DKIM Author Domain Signing Practices (ADSP)
draft-otis-dkim-adsp-03

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Abstract

DomainKeys Identified Mail (DKIM) as described in [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.), defines a domain-level authentication framework for email to permit verification of the source and contents of messages. This document specifies an adjunct mechanism to aid in assessing messages lacking valid DKIM signatures for domains used in the author's address. It defines a record that can advertise the extent that a domain signs outgoing mail publicly exchanged on SMTP port 25, as described in [RFC2821] (Klensin, J., “Simple Mail Transfer Protocol,” April 2001.), and how other hosts can access those records.

Advertisements defined by this document may also increase DKIM signature expectations for messages received by Mail User Agents (MUAs) or for messages which might have been exchanged over protocols other than SMTP. In some circumstances, author domains may wish to have accommodations for protocol failures or for mixed public protocol messaging not made.

In addition, DKIM's identity parameters related to the author address are decisive only when a corresponding DKIM key local-part template precludes an author address. DKIM in conjunction with ADSP is to provide methods for detecting the spoofing of known domains, but not for making assertions about the identity of the message author.

Table of Contents

1.  Introduction
2.  Language and Terminology
    2.1.  Terms Imported from DKIM Signatures Specification
    2.2.  Valid Signature
    2.3.  Valid Author Signature
    2.4.  Key Domain
    2.5.  Author Key Domain
    2.6.  Author Address
    2.7.  Author Domain
    2.8.  Author Domain Signing Practices
3.  Operation Overview
    3.1.  ADSP Discovery Results Usage
    3.2.  ADSP Discovery Results
4.  Detailed Description
    4.1.  DNS Representation
    4.2.  Publication of ADSP Records
5.  IANA Considerations
6.  Security Considerations
    6.1.  ADSP Threat Model
    6.2.  DNS Attacks
    6.3.  DNS Wildcards
7.  References
    7.1.  References - Normative
    7.2.  References - Informative
Appendix A.  Usage Examples
    A.1.  Single Location Domains
    A.2.  Bulk Mailing Domains
    A.3.  Commonly Forged Transactional Messages
    A.4.  Third Party Senders
Appendix B.  Acknowledgements
Appendix C.  Changes in draft-otis-dkim-adsp-00
Appendix D.  Changes in draft-otis-dkim-adsp-01
Appendix E.  Changes in draft-otis-dkim-adsp-02
§  Author's Address
§  Intellectual Property and Copyright Statements

1.  Introduction

DomainKeys Identified Mail (DKIM) defines a mechanism by which email messages can be cryptographically signed, permitting a Key Domain to claim responsibility for the introduction of a message. Receiving hosts can verify the signature by querying the Key Domain to retrieve the appropriate public key, and thereby confirm a message has been attested to by a party in possession of the private key and in control of a portion of the Key Domain.

However, the legacy of the Internet is such that not all messages will be signed or retain a valid signature, and that absence of a valid signature on a message is not an a priori indication of forgery. In fact, during early phases of deployment it is likely that most messages will remain unsigned. However, some domains might decide to sign all of their outgoing mail, for example, to better protect their brand name. It is desirable such domains be able to advertise that fact to other hosts. This is the premise of Author Domain Signing Practices (ADSP).

Receiving hosts implementing this specification ensure greater safety by first inquiring into the validity of the SMTP domain before attempting a series of DKIM related validation transactions. The transactions pertaining to this document determine Author Domain Signing Practices advertised by the Author Domains. This determination is called ADSP Discovery.

The detailed requirements for Author Domain Signing Practices are given in [RFC5016] (Thomas, M., “Requirements for a DomainKeys Identified Mail (DKIM) Signing Practices Protocol,” October 2007.). This document refers extensively to [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.) and assumes the reader is familiar with it.

Requirements Notation:

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.)

2.  Language and Terminology

2.1.  Terms Imported from DKIM Signatures Specification

Some terminology used herein is derived directly from [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.).In several cases, references in that document to Sender have been changed to Author here, to emphasize the relationship to the Author address(es) in the From: header field described in [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.). Briefly,

• A "Local-part" is the part of an address preceding the @ sign, as defined in [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.) and used in [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.).

2.2.  Valid Signature

A Valid Signature is any message signature which correctly verifies using procedures described in section 6.1 of [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.).

2.3.  Valid Author Signature

A Valid Author Signature is any message signature which correctly verifies using procedures described in section 6.1 of [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.), and where the local-part template, the "g" parameter in the key and the Key Domain, matches against the author address.

2.4.  Key Domain

The Key Domain is the domain listed in the d= tag of a Valid Signature.

2.5.  Author Key Domain

The Author Key Domain is the domain listed in the d= tag of a Valid Author Signature that is at or above the Author Domain. The Author Key Domain must match all of its domain components with that of the Author Domain. When a referenced Key contains a t=s tag and value, the Author Key Domain will contain the entire Author Domain for the signature to be valid.

2.6.  Author Address

An "Author Address" is an email address in the From header field of a message [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.). If the From header field contains multiple addresses, the message has multiple Author Addresses.

2.7.  Author Domain

An "Author Domain" is determined by the entire right-hand-side of the Author Address (the portion that is to the right of the "@", excluding the "@" itself).

2.8.  Author Domain Signing Practices

"Author Domain Signing Practices" (or just "practices") consist of a machine-readable record published at the _adsp. subdomain of the Author Domain. The ADSP record includes statements about outgoing mail practices for messages containing the Author Domain.

3.  Operation Overview

Domain owners can publish Author Domain Signing Practices via a distribution service, such as the Domain Name System; specific details related to the use of DNS are given in Section 4.1 (DNS Representation).

Hosts can obtain Author Domain Signing Practices of the domain(s) specified by the Author Domain as described in Section 4.2.2 (Author Signing Practices Discovery Procedure). If a message has multiple Author Addresses, ADSP Discovery SHOULD be performed independently. This standard will not cover the consolidation of combined ADSP Discovery results.

3.1.  ADSP Discovery Results Usage

A receiving host might obtain varying amounts of useful information through ADSP Discovery. Such as:

• When a message has a Valid Author Signature, the ADSP Discovery result is of no benefit since the message is compliant with any possible ADSP assertion.
• When a message has a Valid Signature that is not also a Valid Author Signature, the ADSP Discovery result, in conjunction with the Key Domain of the Valid Signature, is directly relevant to message assessment.
• When a message is without a Valid Author Signature, the ADSP Discovery result at the Author Domain is directly relevant to message assessment.

3.2.  ADSP Discovery Results

Author Domain Signing Practices Discovery at an Author Domain provide three possible results:

• Messages containing the Author Domain advertise practices indicating they do not ensure messages are initially signed by an Author Key Domain.
• Messages containing the Author Domain advertise practices indicating they ensure messages are initially signed by an Author Key Domain.
• Messages containing the Author Domain advertise practices indicating they ensure messages are initially signed, and they recommend dismissing messages not signed by an Author Key Domain.

4.  Detailed Description

4.1.  DNS Representation

Author Signing Practices records are published using the DNS TXT resource record type.

NON-NORMATIVE DISCUSSION [to be removed before publication]: There has been considerable discussion on the DKIM WG mailing list regarding the relative advantages of TXT and a new resource record (RR) type. Read the archive for details.

The RDATA for ADSP resource records is textual in format, with specific syntax and semantics relating to their role in describing Author Domain Signing Practices. The "Tag=Value List" syntax described in section 3.2 of [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.) is used. Records not in compliance with that syntax or the syntax of individual tags described in Section 4.3 MUST be ignored, although they MAY cause the logging of warning messages via an appropriate system logging mechanism. If the RDATA contains multiple character strings, the strings are to be logically concatenated with no delimiters placed between the strings.

The ADSP record for an Author Domain is published at a _adsp. subdomain directly below the Author Domain; e.g., the ADSP record for example.com would be a TXT record that is published at _adsp.example.com. A domain MUST NOT publish more than one ADSP record; the semantics of an ADSP transaction returning multiple ADSP records for a single domain are undefined. (Note that example.com and mail.example.com are different domains.)

4.2.  Publication of ADSP Records

Author Domain Signing Practices are intended to apply to all mail containing the Author Domain. As a defensive strategy against subdomain spoofing, ADSP records can be placed at domains that might appear to support SMTP.

Wildcards within a domain publishing ADSP records will not pose a problem. This is discussed in more detail in Section 6.3 (DNS Wildcards).

4.2.1.  Record Syntax

ADSP records use the "tag=value" syntax described in section 3.2 of [RFC4871] (Allman, E., Callas, J., Delany, M., Libbey, M., Fenton, J., and M. Thomas, “DomainKeys Identified Mail (DKIM) Signatures,” May 2007.). Terms used to describe signing practices employ a metaphor of a door to avoid connotations that might differ from definitions given this document.

Tags used in ADSP records are described below. Unrecognized tags MUST be ignored. In the ABNF below, the WSP token is imported from [RFC2822] (Resnick, P., “Internet Message Format,” April 2001.). The ALPHA and DIGIT tokens are imported from [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.).

dkim=

practices (plain-text; REQUIRED). Possible values are as follows:

OPEN

(Default) The Author Domain practice permits unsigned outbound mail.

CLOSED

The Author Domain practice always initially signs messages containing the Author Domain by an Author Key Domain.

LOCKED

The Author Domain practice always initially signs messages containing the Author Domain by an Author Key Domain. Furthermore, when a message is received without a valid Author Key Domain signature, receiving hosts are requested to dismiss these messages.

ABNF:

      adsp-dkim-tag = %x64.6b.69.6d *WSP "="
          *WSP ("OPEN" / "CLOSED" / "LOCKED")

Unrecognized flags MUST be ignored.

4.2.2.  Author Signing Practices Discovery Procedure

Hosts performing ADSP Discovery should exclude those being made for SMTP clients that have demonstrated a history of abuse. The transactions needed for ADSP Discovery or DKIM signature validation should follow confirmations the Author Domain might support SMTP. In addition, hosts may consider some domains exempt, such as Top Level Domains (TLDs) listed in [RFC2606] (Eastlake, D. and A. Panitz, “Reserved Top Level DNS Names,” June 1999.). TLDs listed in [RFC2606] (Eastlake, D. and A. Panitz, “Reserved Top Level DNS Names,” June 1999.) do not represent a comprehensive list of TLDs that might be excluded from an SMTP domain validation process. Appending to a list of exempted domains may be required.

For the purposes of this section, a "valid ADSP record" is one that is both syntactically and semantically correct; in particular, it matches the ABNF for a tag-list and includes a defined dkim= tag.

• ADSP Discovery. The host SHOULD query DNS for a TXT record corresponding to the Author Domain prefixed by _adsp. (note the trailing dot). The results returned by this operation would be the value of the "dkim" tag or a value of "MISSING" when none exist.
  
  
• NON-NORMATIVE DISCUSSION: Rather than placing ADSP records below the "_domainkey." prefix used by DKIM, "_adsp." prefixed to the Author Domain reduces the number of DNS entities needed when ADSP records are desired at every address record. Delegation of a domain at or below "_domainkey." and at "_adsp." may be required when consolidating control of DNS entries related to DKIM.

When any of the DNS transactions involved in ADSP Discovery result in a temporary error condition, the algorithm terminates without returning a result; possible actions include queuing the message or returning an SMTP error indicating a temporary failure.

NOTE: Within a DNS transaction, as defined by [RFC1034] (Mockapetris, P., “Domain names - concepts and facilities,” November 1987.) section 5.2.2 and [RFC4034] (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “Resource Records for the DNS Security Extensions,” March 2005.) section 3, when a CNAME is returned, the alias name is to be processed as if it were the initial name. [RFC2181] (Elz, R. and R. Bush, “Clarifications to the DNS Specification,” July 1997.) section 10.3 makes an exception for Exchange host names returned by MX records. An Exchange host name must not return a CNAME.

5.  IANA Considerations

ADSP introduces the "_adsp" name into currently unregistered name space. Although domain names beginning with an underscore will not collide with host names, service names for [RFC2782] (Gulbrandsen, A., Vixie, P., and L. Esibov, “A DNS RR for specifying the location of services (DNS SRV),” February 2000.) SRV records, and labels for TXT records defined by other protocols reference underscore prefixed names to designate specific use.

INFORMATIVE NOTE [to be removed before publication]: If at the time of publication no registry has been established or planned for underscore prefixed names, this section may be removed.

6.  Security Considerations

Security considerations in the Author Domain Signing Practices mostly relate to attempts on the part of malicious senders to represent themselves as sending messages from the Author Domain for whom they are not authorized to use in their message, often in an attempt to defraud recipients of the message.

Messages signed by keys having a local-part template in the g= tag restricting the range of valid local-parts are likely employed by systems that are beyond the direct control of the Author Key Domain. As a result, additional care should be taken when the local-part template does not match against the Author Address. Signatures where the g= local-part template does not match against the Author Addresses should not be considered as offering a valid signature.

Additional security considerations regarding Author Domain Signing Practices are found in the DKIM threat analysis (Fenton, J., “Analysis of Threats Motivating DomainKeys Identified Mail (DKIM),” September 2006.) [RFC4686].

6.1.  ADSP Threat Model

Email recipients often have a core set of content Author Domains they trust. Common examples include those of financial institutions with which they have an existing relationship and Internet web transaction sites with which they conduct business. DKIM validation and ADSP Discovery results will not provide any benefit unless receiving hosts act by either treating the message differently during delivery, or by providing some indicator to the end recipient. Such an email annotation system is out of scope for this specification.

Bad actors often seek to exploit the name-recognition of a trusted Author Domain. This might be done with just spoofed display-names or with user local-parts placed above subdomains or cousin domains in the From: header field. This problem is made worse by popular MUAs that do not display actual email addresses. As a result, there is no empirical evidence as to what extent unauthorized use of a domain name contributes to recipient deception, or that its elimination will provide a significant effect. Being able to automate the accrual of behavioural feedback that ignores invalid identifiers better ensures systematic confidence is retained for trusted Author Key Domains.

Nevertheless, training recipients to use automated folder placement could help reduce deceptions that utilize domain look-alike and subdomain based tactics. In addition, automated recognition facilitates optimized processing by receive-side message filtering engines that attempt to curb unauthorized uses of domain names, organizations' names and their logos elsewhere within the message. These attacks and their mitigation are outside the scope of this specification.

The ADSP Discovery algorithm performs one DNS transactions per Author Domain. Since this transaction, as well as those needed to validate the DKIM signature, are driven by domain names in email message headers of possibly fraudulent email, receiving hosts attempting ADSP Discovery and DKIM validation can become participants in traffic multiplication attacks.

These attacks often target servers consolidating and distributing behavioral information aimed at curbing bad-actor activities. An attack may not lead to a denial of service, but may dramatically impact the cost of offering the service. A reduction in those offering consolidated behavioral information places remaining providers in greater jeopardy of receiving a larger portion of the abuse being generated.

6.2.  DNS Attacks

An attack might be waged against DNS infrastructure in an attempt to disable services dependent upon DNS. Such attacks could be made worse by receiving hosts employing ADSP Discovery and DKIM validations. For this reason, SMTP should eventually consider making MX records mandatory for public exchanges. The ADSP Discovery process is not expected to impact the likelihood of an attacker being successful at poisoning local DNS resolvers. In addition, such DNS security issues are addressed by DNSSEC (Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose, “DNS Security Introduction and Requirements,” March 2005.) [RFC4033].

A steady attack may not cause a denial of service, but can consume significant resources related to "in the cloud" consolidation and distribution of behavioral information. A typical strategy used by bad actors employing bot-nets is to rapidly transition from an active to dormant state. The duration of activity experienced by an SMTP server is often brief, and is then followed by a fairly long dormant period. This tactic proves challenging for defensive strategies instantiated by individual hosts. There may be tens of millions of bot-nets in the active state, while hundreds of millions appear dormant to SMTP servers.

Consolidating and distributing behavioral information offers a defensive tactic that can minimize the effectiveness of a blitzkrieg or fast-flux tactic. Unfortunately, often part of a bad-actor's tactic is to inundate behavioral repositories with virtual identifiers. For DKIM, the signature's identity ("i=") parameter can be synthesized since it permits use of wildcarded domains, unlike the Key Domain ("d=") parameter or that of the ADSP record.

Because ADSP operates within the framework of the legacy e-mail system, the default result in the absence of an ADSP record is for the Author Domain to be considered OPEN where not all messages are expected to be signed by a Author Key Domain. It is therefore important that the ADSP clients distinguish a DNS failure such as SERVFAIL from other DNS errors so that appropriate actions can be taken.

It is likely DKIM and ADSP combined roles will be in preventing deception in conjunction with automated folder placements for those domains considered trustworthy. To ensure message reception remains viable for crucial systems when DNS fails, IP addresses of crucial SMTP clients should be white listed to allow ADSP and DKIM to be selectively bypassed during such events.

6.3.  DNS Wildcards

Wildcards within a domain, excluding wildcard MX records, that also publish ADSP records, do not pose a significant problem. While referencing SMTP related records will not provide NXDOMAIN results, SMTP discovery records such as MX or A records offer evidence of SMTP support. Whether AAAA records absent MX or A records are to be considered evidence of SMTP support has not withstood widespread use of AAAA only servers.

NON-NORMATIVE NOTE: Complete ADSP coverage for all subdomains of a domain remains possible. However, ADSP records would need to be published at every subdomain containing A records, in addition to subdomains containing MX records. When SMTP adopts an MX record mandate for public exchanges, ADSP records would then be required only at subdomains containing MX records. This strategy shelters domains not publishing MX records from the transactions associated with any number of Author Addresses and DKIM signatures per message.

7.  References

7.1. References - Normative

7.2. References - Informative

Appendix A.  Usage Examples

These examples are intended to illustrate typical uses of ADSP. They are not intended to be exhaustive, nor to apply to every domain's or mail system's individual situation.

Administrators are advised to consider the ways that mail processing can modify messages in a manner that will invalidate existing DKIM signatures, such as mailing lists, courtesy forwarders, and other paths that could add or modify headers, or modify the message body. In that case, if these modifications invalidate DKIM signatures, receiving hosts will consider the mail not to have an Author Key Domain signature, even though a Valid Author Signature was present when the mail was originally sent.

A.1.  Single Location Domains

A common mail system configuration handles all of a domain's users' incoming and outgoing mail through a single MTA or group of MTAs. In that case, the MTA(s) can be configured to sign outgoing mail with an Author Key Domain signature.

In this situation it might be appropriate to publish a CLOSED ADSP record for the Author Domain, depending on whether users also send mail through other paths that do not apply an Author Key Domain signature. Such paths could include MTAs at hotels or hotspot networks used by travelling users, or web sites that provide "mail an article" features.

A.2.  Bulk Mailing Domains

Another common configuration uses a domain solely for bulk or broadcast mail, with no individual human users, again typically sending all the mail through a single MTA or group of MTAs that can apply an Author Key Domain signature. In this case, before publishing a CLOSED ADSP record, the domain's management should be confident that all of its outgoing mail will be sent through signing MTAs. Lacking individual users, the domain is unlikely to participate in mailing lists, but could still send mail through other paths that might invalidate signatures.

Domain owners also often use specialist mailing providers to send their bulk mail. In that case, the mailing provider needs access to a suitable signing key in order to apply an Author Key Domain signature. One possible method would be for the Author Key Domain owner exchange keys with the mailing provider. Another would be for the Author Key Domain to delegate a subdomain below the _domainkey. label to the mailing provider. For example, bigbank.example might delegate esp-00._domainkey.bigbank.example.com to such a provider. In that case, the provider could generate keys and DKIM DNS records itself and provide Author Key Domain signatures.

A.3.  Commonly Forged Transactional Messages

In some cases, a domain might sign all its outgoing mail with an Author Key Domain signature, but prefers that receiving host systems dismiss mail without a valid Author Key Domain signature to avoid confusion with mail sent from fraudulent sources unable to apply an Author Key Domain signature. (This latter kind of mail is sometimes loosely called "forgeries".) In that case, it might be appropriate to publish a "LOCKED" ADSP record. Note that a domain SHOULD NOT publish a LOCKED ADSP record when it wishes to maximize the likelihood that its mail is delivered, since it could cause some fraction of the mail to become rejected or discarded.

As a special case, if a domain sends no mail at all, it can safely publish a LOCKED ADSP record, since any mail with this Author Domain would be a forgery.

A.4.  Third Party Senders

Another common use case is for a third party to enter into an agreement whereby that third party will send bulk or other mail on behalf of a designated Author Domain, using that domain in the RFC2822 From: or other headers. Due to the many and varied complexities of such agreements, third party signing is not addressed in this specification.

Appendix B.  Acknowledgements

This document was based upon the draft-ietf-dkim-ssp-003. Dave Crocker, Frank Ellermann, and Charles Lindsey inputs were valuable, however inclusion of their names should not be misconstrued as an endorsement of this draft. This draft is an individual submission intended to illustrate a comprehensive solution that might help foreclose protracted debate when there is otherwise general agreement.

Appendix C.  Changes in draft-otis-dkim-adsp-00

• Conditioned Author Signing Practices Discovery Procedure SMTP verification step to be made only when an MX record had not been found.

Appendix D.  Changes in draft-otis-dkim-adsp-01

• Modified the Author Signing Practices Discovery Procedure to better conform with terms in RFC2821. In addition, a note now covers the issue of CNAMEs.

Appendix E.  Changes in draft-otis-dkim-adsp-02

• Modified the abstract to include the language recommended by Dave Crocker, clarified the relationship this document has with DKIM, SMTP and other protocols, and clarified the extent of DKIM identity parameter. The general language describing the intent was taken from the WG charter.
• Included non retention of a valid signature and offered an admonishment to first validate from domain in the introduction.
• Added a separate definition for Valid Author Signatures by including the requirement the local-part template much match against the author addresses.
• Made a few minor changes to the Author Key Domain definition.
• Included the phrase "related to the use of DNS" to the operation Overview as well as expanding upon the term ADSP Discovery in several places.
• Modified ADSP Usage to Discovery Results Usage. The information provided was reorganized from least to most useful.
• Modified the terms in ADSP Discovery Results to be consistent with advertised practices to align more closely with Dave Crocker's Abstract.
• The Record syntax now mentions the terms used are a metaphor for a door, and the terms modified to be in closer alignment with the abstract.
• The ADSP Discovery procedure now warns about unlimited application of this process, and suggests some domains may require exemption, and introduces the term MISSING when no ADSP record is discovered.
• The IANA considerations where shortened based upon the assumption a registry may not be established for underscore prefixed TXT records.
• Change the beginning of the security section to clarify the domain and not the author identity is assured by DKIM and ADSP.
• Changed the wording related to the key "g=" parameter to be more consistent throughout the document.
• Mention in the threat model annotation is required by out of scope.
• Modified the paragraph that describes exploitation of trust to be about the domain and not the author identity.
• Mention that the target of an attack is often directed to behavioral information services.
• Add paragraph describing the typical nature of bot-net behaviour, and how the DKIM "i=" represents a significant venerability for the accrual of behavioral information.
• Add a sentence to highlight benefits using automatic folder placement.
• Expanded the DNS wildcard section to generally describe what might be involved when validating the domains support of SMTP.

Author's Address

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