Internet DRAFT - draft-moore-email-addrquery
draft-moore-email-addrquery
Network Working Group K. Moore
Internet-Draft Network Heretics
Updates: 5231, 6409 (if approved) C. Newman
Intended status: Standards Track Oracle
Expires: January 20, 2016 July 19, 2015
SMTP and SUBMISSION Service Extensions For Address Query
draft-moore-email-addrquery-01.txt
Abstract
This document defines several mechanisms which can be used by a
client such as a Mail User Agent or Mail Submission Agent, to query
an SMTP server which is configured to accept incoming mail for a mail
domain, to obtain information associated with an email address based
in that domain. Among other purposes, these mechanisms are intended
to facilitate discovery of senders' and/or recipients' public keys
for use in automatic verification of whole-message digital signatures
and automatic whole-message encryption of email sent to recipients.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 20, 2016.
Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
Moore & Newman Expires January 20, 2016 [Page 1]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conventions and Terminology Used In This Document . . . . . . 3
3. SMTP Service Extension for Address Query . . . . . . . . . . 4
3.1. AQRY SMTP Command . . . . . . . . . . . . . . . . . . . . 4
3.1.1. Client Use of AQRY command . . . . . . . . . . . . . 5
3.1.2. Normal AQRY Response . . . . . . . . . . . . . . . . 7
3.1.3. Redirect AQRY Response . . . . . . . . . . . . . . . 8
3.1.4. Other response codes . . . . . . . . . . . . . . . . 10
4. Mail Submission Service Extension for Address Query Proxy . . 11
4.1. AQPX Command . . . . . . . . . . . . . . . . . . . . . . 12
4.2. AQPX responses . . . . . . . . . . . . . . . . . . . . . 13
5. Address Query Information Data Model . . . . . . . . . . . . 14
6. Trustworthiness Of Address Query Responses . . . . . . . . . 17
7. Security Considerations . . . . . . . . . . . . . . . . . . . 17
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
8.1. Registration for AQRY SMTP service extension . . . . . . 19
8.2. Registration for AQPX Submission service extension . . . 19
8.3. Registration for new Enhanced Status Codes . . . . . . . 19
8.4. Register new SMTP reply codes . . . . . . . . . . . . . . 22
8.5. Create registry for AQRY data model elements . . . . . . 22
8.6. possibly reserve port number for use in AQRY redirects . 22
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
9.1. Normative References . . . . . . . . . . . . . . . . . . 22
9.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Rationale For Design Choices . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 26
1. Introduction
At least since the introduction of MIME [RFC1321] there has been a
desire to allow message senders to discover capabilities of email
recipients, so that senders could avoid sending message contents to
recipients who were unable to make use of such contents. Similarly,
deployment of per-message encryption (e.g. PEM [RFC1113], S/MIME
[RFC5751], and OpenPGP [RFC4880]) has long been hampered for lack of
a standard and widely supported means to discover and verify
authenticity of senders' and recipients' public key(s).
The issue surfaced recently as part of the DANE working group
discussion in Dallas, and specifically in an effort to adapt TLSA DNS
records [RFC6698] for use in discovery of email recipients' public
Moore & Newman Expires January 20, 2016 [Page 2]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
keys. The problem there was that there is no clean way to map
recipient email addresses onto DNS labels, because the interpretation
of a local-part of an email address is entirely left to the SMTP
server(s) that accept incoming mail for that address's mail domain,
and different mail domains have configured their SMTP servers to
interpret their email addresses in different ways. The "local parts"
of email addresses may be case-sensitive or case-insensitive,
subaddresses may be allowed, there may be some sort of fuzzy
matching, an address may be forwarded elsewhere, and so on. Also,
having public keys for email recipients advertised in DNS would have
facilitated email traffic analysis by an observer watching DNS
queries and responses in cleartext.
Since the knowledge of how to interpret an email address is
inherently embedded in the code and configuration of the SMTP servers
that accept incoming mail for that address's email domain, it appears
that the best way to advertise public keys and other information
associated with email addresses is to do so using the same SMTP
servers that accept such incoming mail. That way, the logic that
maps from address to associated information will be the same logic
that maps from recipient address to recipient mailbox (or forwarding
address). A separate lookup service could be used, but this would
introduce a high probability that the service would interpret the
address differently than that mail domain's SMTP servers, if for no
other reason than configuration errors. However as a compromise for
large mail service providers, and especially those that serve large
numbers of mail domains, the proposed SMTP extension also includes a
"redirect" mechanism that can be used to refer a client to a separate
service which then provides the requested information. Finally, this
document defines an extension to the Mail Submission service which
allows that service to perform an address information lookup
operation on behalf of its authenticated client, which can be useful
to circumvent the common practice of blocking outbound port 25
traffic.
2. Conventions and Terminology Used In This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification expresses syntax using the Augmented Backus-Naur
Form (ABNF) as described in [RFC5234], including the core rules in
Appendix B and rules from [RFC5322].
In examples illustrating protocol interactions, "C:" and "S:"
indicate lines sent by the client and server respectively. If a
single "C:" or "S:" label applies to multiple lines, then the line
Moore & Newman Expires January 20, 2016 [Page 3]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
breaks between those lines are for editorial clarity only and are not
part of the actual protocol exchange.
3. SMTP Service Extension for Address Query
This section defines a service extension to the Simple Message
Transfer Protocol (SMTP) [RFC5321] which can be used by a client to
query the server for information about an email address for which the
server accepts incoming mail.
o The Name of this extension is "Address Query".
o Servers implementing this extension advertise an additional EHLO
keyword of "ADDRQUERY", which has no associated parameters.
o This extension introduces one new SMTP command, AQRY, described
below.
o This extension does not alter any existing SMTP commands, nor does
this extension change the minimum line length that an
implementation of SMTP including this extension must support.
3.1. AQRY SMTP Command
The AQRY SMTP command is used to query an SMTP server about an
address containing a domain name for which the server is configured
to act as a mail exchanger, i.e. to accept incoming mail for
delivery. A SMTP server which accepts incoming mail for a domain is
in a unique position to interpret email addresses containing that
domain, since only such a server can reliably know whether the local
part of that email address is case-sensitive (i.e. whether
Joe@example.com and joe@example.com are distinct users), whether
subaddressing applies to that domain (e.g. whether
joe+xyz@example.com refers to the same user as joe@example.com),
whether a particular recipient has mail forwarded, and so on.
Therefore an SMTP server MUST reject an AQRY command which contains
an address for which the server is not explicitly configured to
accept incoming mail.
In addition, to ensure the integrity of the information provided to
the client and to deter both passive and active attacks, any SMTP
server supporting ADDRQUERY MUST also support the STARTTLS service
extension, and MUST reject any AQRY command not appearing in a TLS-
protected session. Clients using the AQRY command MUST support the
TLS Server Name Indication (SNI) [RFC6066] extension, and MUST supply
the host name of the server to which they wish to connect in the
ServerNameList portion of the extension_data field of the extended
client hello message. (This requirement also applies to Mail
Moore & Newman Expires January 20, 2016 [Page 4]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
Submission servers that implement the Address Query Proxy extension.)
This host name will either be the target of the MX record associated
with the address being queried, or the "host" field as obtained from
an AQRY or AQPX redirect response as defined below. Servers
supporting the Address Query extension SHOULD support SNI and use it
to provide an appropriate server certificate, if available.
The syntax of the AQRY command is as follows:
aqry = "AQRY" SP "<" Mailbox ">"
[ "REFERBY=" address-literal ]
[ "RRVS=" date-time ] [ "COOKIE=" Atom ] CRLF
where address-literal, Atom, and Mailbox are as defined in [RFC5321]
(or if the SMTP server supports the SMTPUTF8 extension, Mailbox is as
defined in [RFC6531]), and date-time is as defined in [RFC3339], with
the added restriction that a "time-secfrac" MUST NOT be used. (XXX
amend the above to use the right nonterminal ABNF symbol for servers
that support SMTPUTF8. Similarly for AQPX command.)
The AQRY command requests that the SMTP server return public
information about the email address ("Mailbox") specified in the
command. If the optional RRVS parameter is included, it specifies
that the email address must have been valid at least since that date
and time. If the server knows that the address has not been valid
that long, it MUST return either an error, or a redirect to a server
that will return an "address not found" error.
(Note: Although the RRVS parameter to the AQRY command has the same
syntax as the RRVS parameter to the RCPT command as defined in
[RFC7293], the two are separate and have different purposes. An SMTP
server MAY support the Address Query extension even if it does not
support the RRVS extension.)
The COOKIE and REFERBY parameters are used only in redirects, as
described below.
(XXX consider max length of COOKIE parameter and whether this affects
minimum SMTP command line length that the server must support.)
3.1.1. Client Use of AQRY command
Clients wishing to query for email address information MUST first
perform a DNS [RFC1035] lookup with query type of MX, specifying the
domain name that appears in the email address. The selection of SMTP
servers among those returned from the DNS query follows the same
algorithm used for selection of SMTP servers to be used for
forwarding mail [RFC5321]: servers with lower MX precedence values
are queried before servers with higher MX precedence values.
Moore & Newman Expires January 20, 2016 [Page 5]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
Clients MUST NOT send an AQRY command to a server that isn't listed
in DNS as a mail exchanger for the mail domain of the address to be
queried. Exception: a client MAY send an AQRY command to an
arbitrary SMTP server without first obtaining that from a DNS MX
lookup, if this is done specifically and entirely for the purpose of
fault diagnosis or configuration checking and the results are not
used to encrypt email nor validate a digital signature.
Note: In contrast to DNS lookups for normal mail routing, the
presence of one or more MX records for the mail domain of the address
being queried is REQUIRED. In particular, the AQPX command described
below, when used without a SERVER argument, will not query an SMTP
server if there are no MX records pointing to it, and only A or AAAA
records.
Clients wishing to use AQRY MUST first negotiate use of TLS
encryption using the STARTTLS command [RFC3207]. If the server does
not advertise STARTTLS, or the TLS negotiation fails, the client MUST
NOT attempt to use AQRY. Furthermore, the client MUST NOT attempt to
use AQRY before first establishing the identity of the server using
the server's certificate, and in particular, that the server's TLS
certificate contains either a DNS-ID (subjectAltName of dNSName type,
see [RFC5280]) or a CN-ID (CN attribute from subject name, see
[RFC6125]) that matches either the DNS name that is the target of the
MX record, or the DNS name appearing in the email address for which
information is being requested. (Exception: the check of the TLS
certificate MAY be skipped if the AQRY operation is done specifically
and entirely for the purpose of fault diagnosis or configuration
checking, and the results are not used to encrypt email nor validate
a digital signature.)
Note: The above rule does not, by itself, establish that the SMTP
server is an authoritative source of information about the
address(es) to be presented to AQRY, because (for example) an MX
record might have been spoofed (unless signed by DNSSEC and the
signature was appropriately verified), or the DNS name associated
with the MX record might not actually have an arrangement with the
SMTP server. However, if the server certificate fails this test,
there's no point in the client doing the AQRY at all. See Section 6.
In response to an AQRY command, the server MUST return one of: a
normal response, a redirect response, or an error response.
A normal response contains information about the email address for
which the request was issued which is specific to that email address,
and/or information about the mail domain name which appears in that
email address. A normal response MAY also contain information such
as address(es) to which incoming mail will be forwarded. In some
Moore & Newman Expires January 20, 2016 [Page 6]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
such cases the client will need to perform additional AQRY
operations, perhaps of other SMTP servers serving other domains, in
order to learn information about the addresses that would eventually
receive mail sent to the originally queried address.
A redirect response does not contain information about the requested
email address, but does contain one or more URLs which may then be
queried to learn about that address and/or its mail domain.
3.1.2. Normal AQRY Response
The normal (non-redirect, non-error) response to a valid AQRY command
consists of multiple lines. Each line but the last line of the
response begins with "212-". The remainder of each line beginning
with "212-" consists of JSON text [RFC7159] subsequently encoded in
BASE64 format as defined in [RFC2045]. BASE64 is used to avoid the
need for the server to produce JSON text which conforms to SMTP line-
length restrictions.
A normal response is not an indication that the address supplied in
the AQRY command is valid. An implementation that does not wish to
disclose whether recipients are valid MAY return "fake" information
in response to AQRY requests for nonexistent recipients. However the
implementation MUST NOT return "fake" information for valid
recipients.
The data structure encoded in the JSON object is further described in
section Section 5.
The last line of the response is of the form:
"212" SP "." CRLF
To produce the normal response to an AQRY command, the server first
produces or obtains the requested information in JSON format. The
server then encodes the entire JSON object using the BASE64
algorithm, such that each line of the BASE64 output does not exceed
76 characters, not including the CRLF character sequence that
terminates each line. The server then prepends "212-" to the
beginning of each line of the BASE64 output. Finally, the server
appends a single line consisting of "212 ." to the output. Per
normal SMTP convention, each line of the reply MUST be terminated by
CRLF.
Note: If a address is configured to forward mail to one or more other
addresses, this can affect the contents of the JSON object or result
in an error. See Section 5.
Moore & Newman Expires January 20, 2016 [Page 7]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
To recover the JSON from the AQRY reply text, the client first
collects the text and ensures that the terminating "212 ." line is
present. The terminating line is then discarded, and the "212-"
prefix is removed from each of the preceding lines. The resulting
text is then fed to the BASE64 decoder to produce a JSON object. The
resulting JSON object may then be interpreted.
3.1.3. Redirect AQRY Response
In the case where the SMTP server is configured to accept incoming
mail for the address presented in the AQRY command, but either of the
following two conditions apply:
(a) in the currently active TLS session, the SMTP server did not
present a server certificate with a subjectAltName with dNSName
type that matches the domain name portion of the email address
presented in the AQRY command; OR
(b) the SMTP server is configured to return a redirect for other
reasons, e.g. to shed load from the SMTP server to another
server which is better equipped to service that kind of query;
the SMTP server MAY return a multi-line redirect response with a
response code of 213. Similar in presentation format to the normal
response, the redirect response consists of BASE64-encoded JSON, with
each line of the BASE64 text preceded by "213-" and the last line of
the response consisting entirely of "213 ." followed by CRLF.
However, the data structure represented in JSON for a redirect
response is different than that of a normal response. The data
structure encoded in a redirect response consists of an array of
objects describing SMTP servers to which the query can be referred.
Each such object may contain the following elements:
host
DNS name, IPv4 address, or IPv6 address of an SMTP server.
port
Optional port number to be used to contact the SMTP server. Port
25 is assumed if this element is not supplied.
cookie
Optional cookie to be passed in the COOKIE parameter to the AQRY
command when querying the server. This parameter may be used for
any purpose by mutual agreement between the server issuing the
redirect response, and the server to which the redirect response
refers. For example: it may be used to encode an encrypted
database record identifier of the named recipient; or it may be
used to encode an encrypted timestamp at which the referral was
Moore & Newman Expires January 20, 2016 [Page 8]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
issued by the server, so that the referred-to server can refuse to
return a response if that timestamp is missing or not recent.
There is no significance to the order in which the list items, or the
elements of any of the objects in the list, appear in the JSON.
Example: A client issues a query for information about
joe@example.com, and the server returns a redirect response:
C: AQRY <joe@example.com>
S: 213-W3siaG9zdCI6ICJmb28uZXhhbXBsZS5jb20iLCAiY29va2llIjogImxranNl
S: 213-b3J1IiwgInBvcnQiOiA5ODc2fSwgeyJob3N0IjogIjEwLjEuMi4zIiwgImNv
S: 213-b2tpZSI6ICJzZndlcnYzMyJ9LCB7Imhvc3QiOiAiMjAwMTpEQjg6YWJjZDo6
S: 213-MToyIiwgImNvb2tpZSI6ICJsa2pzZW9ydSIsICJwb3J0IjogNDMyNX1d
S: 213 .
The client decodes this and obtains the following data structure
(formatted for readability below):
[
{ "host": "foo.example.com", "port": 9876,
"cookie": "lkjseoru" },
{ "host": "10.1.2.3", "cookie": "sfwerv33" },
{ "host": "2001:DB8:abcd::1:2", "port": 4325,
"cookie": "lkjseoru" }
]
The client could then obtain the requested information via any of the
following:
o Open a connection to foo.example.com, port 9876, negotiate
STARTTLS, then issue the command: "AQRY <joe@example.com>
COOKIE=lkjseoru",
o Open a connection to 10.1.2.3, port 25, negotiate STARTTLS, then
issue the command: "AQRY <joe@example.com> COOKIE=sfwerv33", OR
o Open a connection to 2001:DB8:abcd::1:2, port 4325, negotiate
STARTTLS, then issue the command "AQRY <joe@example.com>
COOKIE=lkjseoru" .
In each of the above instances, the client will supply the "host"
parameter from the object as the TLS Server Name Indication (SNI)
HostName. Any RRVS parameter appearing in the original AQRY command
is also supplied when issuing the AQRY command to the redirect
servers. In addition the AQRY REFERBY parameter is supplied with its
value set to the Internet Protocol (v4 or v6) address of the SMTP
server from which the redirect was obtained.
Moore & Newman Expires January 20, 2016 [Page 9]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
Since the SMTP servers returned in a referral response are not
expected to be able to process incoming mail, they are not required
to implement the full SMTP protocol. They need only implement the
following commands: EHLO (advertising STARTTLS and ADDRQUERY),
STARTTLS, AQRY, and QUIT. Such a server SHOULD also implement the
PIPELINING extension. [RFC2920]
3.1.4. Other response codes
In addition to reply codes defined in [RFC5321], the following reply
codes SHOULD be used to indicate the error conditions described
below. In each case below the enhanced status code [RFC5248] that
appears immediately following the 3-digit SMTP reply code is
suggested for use by server implementations supporting the SMTP
ENHANCEDSTATUSCODES extension [RFC2034]. However, the appropriate
status code may depend to some degree on the nature of the SMTP
server implementation or configuration, and there may be cases in
which a different enhanced status code is appropriate. (The SMTP
reply code and enhanced status code serve distinct purposes: The
reply code is intended for use by SMTP clients, and in particular
signals transitions in the SMTP client's state machine. The enhanced
status code is intended for use in Delivery Status Notifications
[RFC3464] and serves as an indication of the likely nature of a
problem with the mail system or network. The relationship between
the two is loose rather than strict.)
IANA NOTE: Some of these codes need to be assigned; these are marked
with IANA- followed by some number. See Section 8. (RFC Editor:
please remove this paragraph on publication.)
411 4.4.3 database lookup temporary failure
This failure occurs whenever the SMTP server must consult some
external database or other service in order to provide the
requested information, and that service fails to respond within a
reasonable time. The client may reasonably retry the command
after some interval. [[XXX specify timeout for AQRY]]
511 5.4.IANA-1 no information available for this address
The address appears to be valid but there is no information
available that is associated with either the address or the mail
domain. This reply code is intended to reflect the case where
there is not actually an error detected on the server, but rather,
a simple absence of information associated with that address and/
or mail domain. If there is some sort of error detected on the
server, say while trying to obtain the requested information from
a separate database, a different reply code and enhanced status
code would be reported.
Moore & Newman Expires January 20, 2016 [Page 10]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
Note: Strictly speaking, this is not an error condition, and would
not normally be assigned a 5xx reply code or 5.y.z enhanced status
code, since there is no requirement that information be available
for every address or mail domain. However, if the client has been
instructed (for example) to not deliver mail without first
encrypting it with the recipient's public key, this is the reply
code that the server should return and (absent some better error-
detection code in the client) the enhanced status code included
with this reply would be reported to the sender as the error which
caused failure of the message to be sent.
513 5.3.IANA-2 service not supported for this domain
The server is configured to accept incoming mail for the domain
name appearing in the address, but the server is not configured to
perform queries for addresses in that domain.
550 5.1.1 no such address
The address does not exist. Note: Depending on the specific
nature of the error, there are several enhanced status codes that
could reasonably be used with the 550 reply code in response to
AQRY, including 5.1.1, 5.5.4, 5.6.7, and others. [[XXX should
probably explain when it's appropriate to use other SMTP reply
codes than those listed in this document, either that or list a
few more valid responses.]]
557 5.3.IANA-3 server does not accept incoming mail for this domain
The server is not configured to accept incoming mail for the
domain name appearing in the address.
523 5.7.IANA-4 TLS required but not negotiated
This reply code is returned whenever a client attempts an AQRY
command in a SMTP session that is not protected by TLS.
4. Mail Submission Service Extension for Address Query Proxy
This section defines a service extension to the Mail Submission
Protocol [RFC6409] which can be used by an authenticated, authorized
client to query an SMTP server on port 25 for information about an
email address. This is intended only as a workaround for port 25
blocking, so the extension is minimally tailored for that purpose.
o The Name of this extension is "Address Query Proxy".
o Servers implementing this extension advertise an additional EHLO
keyword of "ADDRQUERYPROXY", which has no associated parameters.
o This extension introduces one new Submission command, AQPX,
described below.
Moore & Newman Expires January 20, 2016 [Page 11]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
o This extension does not alter any existing Submission service
commands, nor does this extension change the minimum line length
that an implementation of the Submission protocol including this
extension must support.
4.1. AQPX Command
The AQPX command is used to query an Submission server for
information about an email address. The client user MUST have
already been authenticated and verified to be authorized to use that
Submission server. Use of this command by a mail client (such as a
Mail User Agent) is OPTIONAL; this specification does not prohibit a
client directly contacting an SMTP server. However, it is expected
that clients will often need a service as a workaround for the common
practice of blocking outbound traffic on TCP port 25.
The AQRY command requires a TLS-protected session, either by using a
server port that automatically establishes TLS on connect, or by
using a cleartext port and the STARTTLS command. Clients MUST NOT
attempt to use the AQRY command if the session is not protected with
TLS; and servers MUST refuse an AQRY command that appears in a
session not protected with TLS.
When this command is received, the Submission server will then:
o verify that the user is authenticated via a TLS-protected session
o consult the SMTP server specified in the AQPX command,
o negotiate a TLS session using STARTTLS,
o verify that the server's certificate is valid and has an
appropriate subjectAltName for the address, and if so,
o issue an AQRY command to that server, and
o return the response from the AQRY command.
If some error occurs in the process of performing the above, the
Submission server will return an appropriate response code.
The syntax of the AQPX command is as follows:
aqpx = "AQPX" SP "<" Mailbox ">"
[ "SERVER=" ( Domain
/ IPv4-address-literal
/ IPv6-address-literal) ]
[ "RRVS=" date-time ] [ "COOKIE=" Atom ] CRLF
Moore & Newman Expires January 20, 2016 [Page 12]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
where IPv4-address-literal, IPv6-address-literal, Atom, Domain, and
Mailbox are as defined in [RFC5321] (or if the Submission server
supports the SMTPUTF8 extension, Domain and Mailbox are as defined in
[RFC6531]), and date-time is as defined in [RFC3339] with the added
restriction that a "time-secfrac" MUST NOT be used.
The optional SERVER parameter specifies an SMTP server to consult.
Since this may be any server included in either a response to a DNS
MX query, or a server returned in a redirect from a previous query to
an SMTP server, the Submission server SHOULD NOT restrict the servers
to which a client may issue a query. There is no provision for
specifying the port at which the SMTP server is to be contacted; the
client is assumed to be able to directly contact servers on ports
other than 25. If no SERVER parameter is supplied, the Submission
server will perform an MX lookup of the domain portion of the
address, and attempt to issue the AQRY command to one or more servers
(if any are found) in order of increasing precedence until it either
receives a result that is not a temporary failure; that result is
returned to the Submission client. Regardless of whether a SERVER
parameter is specified, Submission servers SHOULD implement a
reasonable timeout for obtaining the information necessary to respond
to the AQPX command. If the timeout expires, the server should
return a 431 error (see below).
The RRVS and COOKIE parameters are passed to the AQRY command issued
to the SMTP server.
4.2. AQPX responses
Since this is a proxy service that is intended to return a response
from a remote SMTP server, any valid response to the SMTP AQRY
command (including a normal response, redirect response, or error
response) is also a valid response to a Submission service AQPX
command.
The submission service SHOULD NOT follow redirects returned by an
SMTP server, and MUST return the SMTP server's response intact and
without modification.
In addition, the following AQPX-specific response codes are
permitted:
o 431 4.4.2 connection or query to remote SMTP server timed out
o 541 5.7.IANA-5 invalid remote server certificate
o 542 5.7.IANA-6 server certificate for <smtp-server-name> does not
match <domain>
Moore & Newman Expires January 20, 2016 [Page 13]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
o 43x 4.x.y DNS query timed out
o 5xx 5.x.y No MX records found
5. Address Query Information Data Model
Note: This section is preliminary and is expected to require
considerable work, and to be moved to a separate document.
XXX consider using JSON Web Signature (JWS) as an optional means of
authenticating returned information.
The response to the AQRY command is a single JSON object. This JSON
object contains zero or more members, each of which is itself an
object. The members of the top-level object either supply
information about a mail domain, or a specific email address. Mail
domain objects are named using the DNS name of their mail domain
(which does not contain an "@"), while email address objects are
named for their email address (which does contain an "@"). In either
case the domain or email address used to name the second-level
objects are in the same format as would be presented to a SMTP MAIL
command. (i.e. If the SMTP server supports the SMTPUTF8 extension
[RFC6531], the address MAY be in UTF-8; otherwise the address MUST be
in ASCII).
Both mail domain objects and email address objects are "flat", that
is to say, the members of these objects are either strings, numbers,
booleans, or arrays whose members consist exclusively of one or more
of these. For ease of use in some programming languages, the names
of the elements of both mail domain and email address objects MUST
begin with an ASCII letter ("a"-"z" or "A"-"Z") and MUST consist only
of letters, digits, and underscore ("_").
The requirement for a flat structure is to discourage creation of
complex data models to represent mail domain and address information.
(Note in draft: this is subject to change, but it seems to one of the
authors that one result of the ability to define very complex
structures to represent information is that there is a resulting
tendency to model information using more complexity than is useful or
helpful. Having a relatively simple data model for representation of
such information may also make it easier to store and manipulate such
data in existing SMTP implementations that are implemented in a
variety of programming langauges, and in existing databases that are
used to store address validity and forwarding information.
The objects included in the AQRY response are expected to provide
information about the domain and/or email address supplied in the
AQRY command. However, multiple domain and/or email address objects
Moore & Newman Expires January 20, 2016 [Page 14]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
MAY be included if the information is relevant and potentially useful
to the client, and the server is authoritative for such information.
For instance, if joe@a.example.com has his mail forwarded to
bob@b.example.com, the response to AQRY of joe@a.example.com MAY
include domain objects for both "a.example.com" and "b.example.com",
and email address objects for both "joe@a.example.com" and
"bob@b.example.com". However, for this information to be useful to
the client, the server's TLS certificate SHOULD include DNS-ID
attributes matching both "a.example.com" and "b.example.com" (whether
or not wildcard objects are used), and the returned information for
"joe@a.example.com" SHOULD reveal that that address is being
forwarded to "bob@b.example.com". (Note: Encryption of mail to be
forwarded is tricky to get right for various reasons, including that
a recipient may not wish to publicly reveal his forwarding
address(es), and also that a sender may not wish his encrypted mail
to be encrypted for, and forwarded to, one or more different persons
or addresses than the originally-specified recipient.)
Examples of attributes that might appear within mail domain objects
might include:
transmit_signing_policy
A string describing the policy with which messages originated by
addresses at this email domain are signed by the domain's mail
submission service, if not signed by the sender of the message.
e.g. "always", "when-able" (only when the recipient advertises
support for a signature algorithm that the sending domain
supports), "only-by-sender" (messages are only signed when
presented to the submission service already signed by the sender's
MUA), "on-sender-request" (the submission service will sign a
message if requested to do so by the sender's MUA), "never". This
information could be used by a recipient to determine whether a
particular received message should have been signed. (XXX However
since this policy can vary over time, this doesn't help when
looking at an old message.)
transmit_signing_keys
An array of [ keytype, key ] pairs, where keytype is a string, and
key is a representation of the public key used to sign outgoing
messages.
transmit_encryption_policy
Describes the policy by which outgoing messages are encrypted to
be read by recipients. One of: "always", "when-able", "only-by-
sender", "on-sender-request", "never". This information could be
used by a recipient to determine whether a particular message
should have been encrypted. (But see the note above about time
sensitivity.)
Moore & Newman Expires January 20, 2016 [Page 15]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
transmit_encryption_passthrough
Either true or false depending on whether the submission service
will permit already-encrypted messages to be submitted.
receive_accept_encryption
A list of encryption formats/algorithms which the domain itself
can decrypt on behalf of its recipients, if the message is
encrypted using the domain's public key.
receive_encryption_passthrough
Either true or false depending on whether received encrypted
messages that were encrypted for the recipient's key, rather than
the mail domain's key, will be accepted and be passed to the
recipient's message store in encrypted form.
receive_encryption_forwarding_passthrough
Similar to the above, but describes whether encrypted messages may
be forwarded in encrypted form to the recipient's forwarding
address(es).
Examples of attributes that might appear within mail address objects
include:
sender_signing_policy
Describes the conditions in which the sender signs outgoing mail.
sender_signing_key_list
A set of [format, key] pairs, where format is a string describing
the format and signature algorithm, and key is the public key in
an appropriate format for that signature format. There may be
multiple keys with the same format string.
sender_encryption_policy
Describes the conditions in which the sender encrypts outgoing
mail.
recipient_accept_encryption
Describes the encrypted message formats accepted by the recipient.
recipient_decryption_key_list
A set of [ format, key ] pairs listing the message formats and
public keys for which the recipient is able to decrypt mail.
recipient_accept_signature
A list of signed message formats that the recipient can
potentially verify.
recipient_forwarding_addresses
Moore & Newman Expires January 20, 2016 [Page 16]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
A list of forwarding addresses that the recipient wishes to
publicly disclose.
6. Trustworthiness Of Address Query Responses
As described above, the JSON object returned in a normal AQRY
response may itself contain multiple member objects, each providing
information about a separate email address or mail domain. The
trustworthiness of each member MUST be evaluated separately.
A member object of a normal AQRY response MUST NOT be considered
trustworthy for any purpose, unless the TLS server certificate used
to authenticate the session in which the information was obtained
contained a DNS-ID identifier (subjectAltName of dNSName type
[RFC5280]) or a CN-ID (CN attribute from subject name, [RFC6125])
specifying a dNSName matching either the domain used to name the
section, or the domain portion of the email address used to name the
section.
A redirect AQRY response that does not meet the above criteria (i.e
neither any DNS-ID nor the CN-ID from the server's certificate
matches the domain name of the address presented to AQRY) MAY be used
to identify redirect servers. However, the above certificate checks
MUST be applied when consulting redirect servers and determining the
trustworthiness of their results. In other words, while it's
acceptable for the mail exchangers for a mail domain that are listed
in DNS to not have certificates that match that mail domain, it's not
acceptable for the redirect AQRY servers for that mail domain to have
have certificates that match that mail domain.
A Submission server implementing the AQPX extension MUST evaluate the
trustworthiness of each named object in the response and only return
those sections which are verified to be trustworthy according to the
above rule.
A Submission client using the AQPX extension MUST follow the
certificate checking rules in [I-D.ietf-uta-email-tls-certs].
7. Security Considerations
o This service relies on the SMTP server's TLS server certificate to
authenticate per-domain and per-address information, including
potentially public keys for use with encryption and digital
signatures. While it appears to have the advantage of being
deployable, as most service providers will already be familiar
with TLS and X.509 certificate management, the Address Query
service may invest more trust in such servers and their key
management practices than was designed for.
Moore & Newman Expires January 20, 2016 [Page 17]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
o The Address Query Proxy extension to the Submission service
inherently requires the client and user to trust the Submission
server to do correct validation and correct name matching of SMTP
servers' certificates, as there is no good way to transfer the
integrity and authenticity assurances provided by the TLS protocol
to the Submission server from the remote SMTP server, to the
Submission client.
o With AQRY as it's currently specified, a mail service provider
supporting multiple client domains will either need to manage
multiple certificates and private keys (one or more for each
client domain), or refer queries to a separate server, managed by
the client, for each client. A client may prefer to not expose
its private key to a mail service provider. However this choice
MAY be made on a per-client basis.
o Especially since the Address Query SMTP service extension does not
require authentication, and since it may potentially provide
arbitrary information about an email address or mail domain,
attackers may attempt to use it to "mine" or "harvest" information
about arbitrary mail addresses and their users. It is recommended
that only the minimum information necessary for the desired level
of mail operation be exposed through this service. In addition,
servers MAY return "fake" information for nonexistent recipients
in order to discourage probing of arbitrary addresses. Servers
MAY also implement rate limiting of AQRY command processing,
though this may not be effective against distributed information
gathering networks.
o As compared to most uses of SMTP and Submission protocols which
primarily transmit data from client to server, this extension is
specifically designed to transmit potentially-significant amounts
of data from the server to the client. As such, client
implementations MUST NOT fail or corrupt internal data when
receiving large amounts of data in an SMTP response, nor when
processing the returned data (whether or not correctly formatted).
o There is some potential for the AQPX Submission service extension
to be used as a means of traffic laundering when attacking other
services. However this potential is believed to be minimal
(except for data harvesting attack described above) as this
service only communicates with other hosts on TCP port 25 and is
limited to a very specific SMTP command sequence. Submission
servers MUST require authentication before accepting AQPX
commands, SHOULD implement rate limiting of such commands or other
mechanism to prevent single clients from overusing the service,
and SHOULD log at least the number of AQPX queries on a per-user
basis.
Moore & Newman Expires January 20, 2016 [Page 18]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
o The proposed data model anticipates this service being useful for
any of several modes of per-message encryption. In addition to
end-to-end encryption (in which encryption is done by the sender's
MUA and decryption is done by the recipient's MUA), it is also
possible for encryption to be done by the sender's Submission
agent, or for the decryption to be done by the recipient's SMTP
server or delivery agent or message store (if the sender's MUA or
Submission agent encrypt the message for the recipient domain's
encryption key). While end-to-end encryption is in some sense the
ideal situation, as it theoretically minimizes the potential for
exposure of messages, there are several "real world" barriers to
its universal adoption. One such barrier is that the majority of
mail users today use webmail services in which end-to-end is a
fairly meaningless concept. Another such barrier is the
widespread use of spam filters and message filtering firewalls
which require exposure to received messages in cleartext to be
useful. Another such barrier is the legal or other requirement
that many organizations have for archival of email communications.
Finally, many kinds of personal computer are notoriously insecure,
so a user's messages and credentials might actually be better
protected on a well-managed server than on his or her own PC. By
permitting flexibility in how email encryption is done it is hoped
that encryption may be more widely deployed and that it will
provide an upgrade path to optimal security for everyone.
8. IANA Considerations
(this section requires elaboration.)
8.1. Registration for AQRY SMTP service extension
(to be written)
8.2. Registration for AQPX Submission service extension
(to be written)
8.3. Registration for new Enhanced Status Codes
Code: X.4.IANA-1 (XXX replace IANA-1)
Sample Text: no information available for this address
Associated Status Code(s): 511
Description: This code is used in response to an AQRY command when
the server has no information associated with an address. This
condition is distinct from a temporary condition such as the
Moore & Newman Expires January 20, 2016 [Page 19]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
server being unable to contact a database to obtain such
information.
Reference: (XXX this document)
Submitter: K. Moore, C. Newman
Change controller: IESG
Code: X.3.IANA-2 (XXX replace IANA-2)
Sample Text: service not supported for this domain
Associated Status Code(s): 513
Description: The server is configured to accept incoming mail for
the domain name appearing in the address, but the requested
service is not supported for that domain. Used, for instance, in
response to the AQRY command.
Reference: (XXX this document)
Submitter: K. Moore, C. Newman
Change controller: IESG
Code: X.3.IANA-3 (XXX replace IANA-3)
Sample Text: incoming mail not accepted for this domain
Associated Status Code(s): 557
Description: This server does not accept incoming mail for the
domain in the supplied address. Originally intended for use in
response to AQRY command. Should not be used in response to RCPT
command because there is a long history of RCPT returning other
response codes for this condition.
Reference: (XXX this document)
Submitter: K. Moore, C. Newman
Change controller: IESG
Code: X.4.IANA-4 (XXX replace IANA-4)
Sample Text: TLS required but not negotiated
Moore & Newman Expires January 20, 2016 [Page 20]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
Associated Status Code(s): 523
Description: The requested service (which was not an authentication
command) is required to be issued within an authenticated TLS
session, but was not issued within such a session.
Reference: (XXX this document)
Submitter: K. Moore, C. Newman
Change controller: IESG
Code: X.4.IANA-5 (XXX replace IANA-5)
Sample Text: invalid remote server certificate
Associated Status Code(s): 541
Description: This code is used when the requested service must
consult with some remote service to fulfill its function, and the
remote server did not provide a valid TLS server certificate that
matched its domain name. Originally used with AQPX Submission
service command.
Reference: (XXX this document)
Submitter: K. Moore, C. Newman
Change controller: IESG
Code: X.4.IANA-6 (XXX replace IANA-6)
Sample Text: service certificate provided by <server-name> does not
match <domain>
Associated Status Code(s): 542
Description: This code is used when the requested service must
consult some remote service to fulfill its function, and the
certificate provided by the remote service was not correct to
establish the authenticity of the requested information.
Reference: (XXX this document)
Submitter: K. Moore, C. Newman
Change controller: IESG
Moore & Newman Expires January 20, 2016 [Page 21]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
8.4. Register new SMTP reply codes
XXX
8.5. Create registry for AQRY data model elements
XXX
8.6. possibly reserve port number for use in AQRY redirects
XXX
9. References
9.1. Normative References
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<http://www.rfc-editor.org/info/rfc2045>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC2920] Freed, N., "SMTP Service Extension for Command
Pipelining", STD 60, RFC 2920, DOI 10.17487/RFC2920,
September 2000, <http://www.rfc-editor.org/info/rfc2920>.
[RFC3207] Hoffman, P., "SMTP Service Extension for Secure SMTP over
Transport Layer Security", RFC 3207, DOI 10.17487/RFC3207,
February 2002, <http://www.rfc-editor.org/info/rfc3207>.
[RFC3339] Klyne, G. and C. Newman, "Date and Time on the Internet:
Timestamps", RFC 3339, DOI 10.17487/RFC3339, July 2002,
<http://www.rfc-editor.org/info/rfc3339>.
[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<http://www.rfc-editor.org/info/rfc5234>.
Moore & Newman Expires January 20, 2016 [Page 22]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
[RFC5280] Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
Housley, R., and W. Polk, "Internet X.509 Public Key
Infrastructure Certificate and Certificate Revocation List
(CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
<http://www.rfc-editor.org/info/rfc5280>.
[RFC5321] Klensin, J., "Simple Mail Transfer Protocol", RFC 5321,
DOI 10.17487/RFC5321, October 2008,
<http://www.rfc-editor.org/info/rfc5321>.
[RFC5322] Resnick, P., Ed., "Internet Message Format", RFC 5322,
DOI 10.17487/RFC5322, October 2008,
<http://www.rfc-editor.org/info/rfc5322>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<http://www.rfc-editor.org/info/rfc6066>.
[RFC6125] Saint-Andre, P. and J. Hodges, "Representation and
Verification of Domain-Based Application Service Identity
within Internet Public Key Infrastructure Using X.509
(PKIX) Certificates in the Context of Transport Layer
Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
2011, <http://www.rfc-editor.org/info/rfc6125>.
[RFC6409] Gellens, R. and J. Klensin, "Message Submission for Mail",
STD 72, RFC 6409, DOI 10.17487/RFC6409, November 2011,
<http://www.rfc-editor.org/info/rfc6409>.
[RFC7159] Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
Interchange Format", RFC 7159, DOI 10.17487/RFC7159, March
2014, <http://www.rfc-editor.org/info/rfc7159>.
[I-D.ietf-uta-email-tls-certs]
Melnikov, A., "Updated TLS Server Identity Check Procedure
for Email Related Protocols", draft-ietf-uta-email-tls-
certs-03 (work in progress), June 2015.
9.2. Informative References
[RFC1113] Linn, J., "Privacy enhancement for Internet electronic
mail: Part I - message encipherment and authentication
procedures", RFC 1113, DOI 10.17487/RFC1113, August 1989,
<http://www.rfc-editor.org/info/rfc1113>.
Moore & Newman Expires January 20, 2016 [Page 23]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
[RFC1321] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
DOI 10.17487/RFC1321, April 1992,
<http://www.rfc-editor.org/info/rfc1321>.
[RFC2034] Freed, N., "SMTP Service Extension for Returning Enhanced
Error Codes", RFC 2034, DOI 10.17487/RFC2034, October
1996, <http://www.rfc-editor.org/info/rfc2034>.
[RFC3464] Moore, K. and G. Vaudreuil, "An Extensible Message Format
for Delivery Status Notifications", RFC 3464,
DOI 10.17487/RFC3464, January 2003,
<http://www.rfc-editor.org/info/rfc3464>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<http://www.rfc-editor.org/info/rfc4880>.
[RFC5248] Hansen, T. and J. Klensin, "A Registry for SMTP Enhanced
Mail System Status Codes", BCP 138, RFC 5248,
DOI 10.17487/RFC5248, June 2008,
<http://www.rfc-editor.org/info/rfc5248>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <http://www.rfc-editor.org/info/rfc5751>.
[RFC6531] Yao, J. and W. Mao, "SMTP Extension for Internationalized
Email", RFC 6531, DOI 10.17487/RFC6531, February 2012,
<http://www.rfc-editor.org/info/rfc6531>.
[RFC6698] Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
of Named Entities (DANE) Transport Layer Security (TLS)
Protocol: TLSA", RFC 6698, DOI 10.17487/RFC6698, August
2012, <http://www.rfc-editor.org/info/rfc6698>.
[RFC7293] Mills, W. and M. Kucherawy, "The Require-Recipient-Valid-
Since Header Field and SMTP Service Extension", RFC 7293,
DOI 10.17487/RFC7293, July 2014,
<http://www.rfc-editor.org/info/rfc7293>.
Appendix A. Rationale For Design Choices
This section is not normative.
o As described above, the choice of using an SMTP extension for this
purpose, and using mail exchangers for the authoritative sources
Moore & Newman Expires January 20, 2016 [Page 24]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
of this information, resulted from the observation that only the
SMTP servers for incoming mail for a mail domain reliably know how
to interpret an email address from that mail domain.
o The redirect response was included because many mail service
providers accept incoming mail for large numbers of mail domains,
and that it is infeasible and generally inappropriate for a large
mail service provider to maintain server certificates that name
each of the mail domains for which it provides service. The
redirect response thus permits referral of a request to a specific
server for each mail domain. The redirect response also may be
useful in the case where the listed MX servers for a mail domain
do not handle incoming mail directly, but rather forward it to or
through one or more internal servers (e.g. firewalls, spam
filters) before the message reaches the server responsible for
address interpretation and delivery. Finally, the redirect
response may be useful in allowing a heavily-loaded server to
devote its resources to mail delivery by referring queries about
email address information elsewhere.
o Originally the redirect response contained https URLs, and the
queries to other servers were to use http/1 or http/2. This
appeared to make client implementations unnecessarily complex, for
several reasons: differences in error reporting between SMTP and
HTTP required two sets of error codes and different logic on the
client side for each, the existence of HTTP redirects coupled with
the need to verify subjectAltName in server certificates appeared
to make it difficult to reuse ordinary HTTP library routines. So
redirects were changed to specify the DNS name or address, and
port, of one or more SMTP servers, thus allowing reuse of the same
code on the client for both kinds of query.
o As indicated above, the Submission extension was created as a
workaround for the common practice of blocking outbound TCP
traffic to a destination port of 25. However, it also seemed
appropriate for a Submission server to support this functionality
based on an anticipated desire for a Submission server to support
additional extensions (not defined in this document) for server-
side signing and/or encryption of submitted mail.
o The Submission AQPX command doesn't support arbitrary ports
because it seemed like too much of an opportunity for clients to
use that facility for malicious purposes, even if the clients do
have to be authenticated. It might be worth considering reserving
a specific port for SMTP AQRY referrals.
o The Submission AQPX command doesn't handle MX lookup, referrals,
or retries because of concern over timeout hazards, and because it
Moore & Newman Expires January 20, 2016 [Page 25]
Internet-Draft SMTP/SUBMISSION Email Address Query July 2015
seemed better to let clients perform these operations than to
burden servers with them.
Authors' Addresses
Keith Moore
Network Heretics
PO Box 1934
Knoxville, TN 37901
US
Email: moore@network-heretics.com
Chris Newman
Oracle
440 E. Huntington Dr., Suite 400
Arcadia, CA 91006
US
Email: chris.newman@oracle.com
Moore & Newman Expires January 20, 2016 [Page 26]
