Request for Comments: 3865 Memory Palace Press
Category: Standards Track September 2004
A No Soliciting Simple Mail Transfer Protocol (SMTP)
Service Extension
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document proposes an extension to Soliciting Simple Mail
Transfer Protocol (SMTP) for an electronic mail equivalent to the
real-world "No Soliciting" sign. In addition to the service
extension, a new message header and extensions to the existing
"received" message header are described.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. The Spam Pandemic. . . . . . . . . . . . . . . . . . . . 3
1.2. No Soliciting in the Real World. . . . . . . . . . . . . 4
1.3. No Soliciting and Electronic Mail. . . . . . . . . . . . 5
2. The No-Soliciting SMTP Service Extension . . . . . . . . . . . 6
2.1. The EHLO Exchange. . . . . . . . . . . . . . . . . . . . 7
2.2. Solicitation Class Keywords. . . . . . . . . . . . . . . 7
2.2.1. Note on Choice of Solicitation Class Keywords. . 8
2.3. The MAIL FROM Command. . . . . . . . . . . . . . . . . . 9
2.4. Error Reporting and Enhanced Mail Status Codes . . . . . 10
2.5. Solicitation Mail Header . . . . . . . . . . . . . . . . 10
2.6. Insertion of Solicitation Keywords in Trace Fields . . . 11
2.7. Relay of Messages. . . . . . . . . . . . . . . . . . . . 12
2.8. No Default Solicitation Class. . . . . . . . . . . . . . 12
3. Security Considerations . . . . . . . . . . . . . . . . . . . 13
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
4.1. The Mail Parameters Registry . . . . . . . . . . . . . . 13
4.2. Trace Fields . . . . . . . . . . . . . . . . . . . . . . 14
4.3. The Solicitation Mail Header . . . . . . . . . . . . . . 14
5. Author’s Acknowledgements . . . . . . . . . . . . . . . . . . 14
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
6.1. Normative References . . . . . . . . . . . . . . . . . . 15
6.2. Informative References . . . . . . . . . . . . . . . . . 15
Appendix A. Collected ABNF Descriptions (Normative) . . . . . . . 18
Author’s Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction
1.1. The Spam Pandemic
Unsolicited Bulk Email (UBE), otherwise known as spam, has become as
one of the most pressing issues on the Internet. One oft-quoted
study estimated that spam would cost businesses $13 billion in 2003
[Ferris]. In April 2003, AOL reported that it had blocked 2.37
billion pieces of UBE in a single day [CNET]. And, in a sure sign
that UBE has become of pressing concern, numerous politicians have
begun to issue pronouncements and prescriptions for fighting this
epidemic [Schumer][FTC].
A variety of mechanisms from the technical community have been
proposed and/or implemented to fight UBE:
o Whitelists are lists of known non-spammers. For example, Habeas,
Inc. maintains a Habeas User List (HUL) of people who have agreed
to not spam. By including a haiku in email headers and enforcing
copyright on that ditty, they enforce their anti-spamming terms of
service [Habeas].
o Blacklists are lists of known spammers or ISPs that allow spam
[ROKSO].
o Spam filters run client-side or server-side to filter out spam
based on whitelists, blacklists, and textual and header analysis
[Assassin].
o A large number of documents address the overall technical
considerations for the control of UBE [crocker-spam-techconsider],
operational considerations for SMTP agents [RFC2505], and various
extensions to the protocols to support UBE identification and
filtering [danisch-dns-rr-smtp][daboo-sieve-spamtest][crouzet-
amtp].
o Various proposals have been advanced for "do not spam" lists, akin
to the Federal Trade Commission’s "Do Not Call" list for
telemarketers [FTC.TSR].
Terminology
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 BCP 14, RFC 2119
[RFC2119].
1.2. No Soliciting in the Real World
Municipalities frequently require solicitors to register with the
town government. And, in many cases, the municipalities prohibit
soliciting in residences where the occupant has posted a sign. The
town of West Newbury, Massachusetts, for example, requires:
"It shall be unlawful for any canvasser or solicitor to enter the
premises of a resident or business who has displayed a ’No
Trespassing’ or ’No Soliciting’ sign or poster. Further, it shall
be unlawful for canvassers or solicitors to ignore a resident or
business person’s no solicitation directive or remain on private
property after its owner has indicated that the canvasser or
solicitor is not welcome" [Newbury].
Registration requirements for solicitors, particularly those
soliciting for political or religious reasons, have been the subject
of a long string of court cases. However, the courts have generally
recognized that individuals may post "No Soliciting" signs and the
government may enforce the citizen’s desire. In a recent case where
Jehovah’s Witnesses challenged a registration requirement in the city
of Stratton, Connecticut, saying they derived their authority from
the Scriptures, not the city. However, the court noted:
"A section of the ordinance that petitioners do not challenge
establishes a procedure by which a resident may prohibit
solicitation even by holders of permits. If the resident files a
’No Solicitation Registration Form’ with the mayor, and also posts
a ’No Solicitation’ sign on his property, no uninvited canvassers
may enter his property... " [Watchtower].
Even government, which has a duty to promote free expression, may
restrict the use of soliciting on government property. In one case,
for example, a school district was allowed to give access to its
internal electronic mail system to the union that was representing
teachers, but was not required to do so to a rival union that was
attempting to gain the right to represent the teachers. The court
held that where property is not a traditional public forum "and the
Government has not dedicated its property to First Amendment
activity, such regulation is examined only for reasonableness"
[Perry].
The courts have consistently held that the state has a compelling
public safety reason for regulating solicitation. In Cantwell v.
Connecticut, the Supreme Court held that "a State may protect its
citizens from fraudulent solicitation by requiring a stranger in the
community, before permitting him publicly to solicit funds for any
purpose, to establish his identity and his authority to act for the
cause which he purports to represent" [Cantwell]. And, in Martin v.
City of Struthers, the court noted that "burglars frequently pose as
canvassers, either in order that they may have a pretense to discover
whether a house is empty and hence ripe for burglary, or for the
purpose of spying out the premises in order that they may return
later" [Martin]. The public safety issue applies very much to email,
where viruses can easily be delivered, in contrast to telephone
solicitations where public safety is not nearly as much an issue.
This analysis is U.S.-centric, which is partly due to the background
of the author. However, the concept of prohibiting unwanted
solicitation does carry over to other countries:
o In Hong Kong, offices frequently post "no soliciting" signs.
o In the United Kingdom, where door-to-door peddlers are fairly
common, "no soliciting" signs are also common.
o In Australia, where door-to-door does not appear to be a pressing
social problem, there was legislation passed which outlawed the
practice of placing ads under wipers of parked cars.
o In France, which has a long tradition of door-to-door
solicitation, apartment buildings often use trespass laws to
enforce "no solicitation" policies.
o In the Netherlands, where door-to-door solicitation is not a
pressing issue, there is a practice of depositing free
publications in mailboxes. The postal equivalent of "no spam"
signs are quite prevalent and serve notice that the publications
are not desired.
1.3. No Soliciting and Electronic Mail
Many of the anti-spam proposals that have been advanced have great
merit, however none of them give notice to an SMTP agent in the
process of delivering mail that the receiver does not wish to receive
solicitations. Such a virtual sign would serve two purposes:
o It would allow the receiving system to "serve notice" that a
certain class of electronic mail is not desired.
o If a message is properly identified as belonging to a certain
class and that class of messages is not desired, transfer of the
message can be eliminated. Rather than filtering after delivery,
elimination of the message transfer can save network bandwidth,
disk space, and processing power.
This memo details a series of extensions to SMTP that have the
following characteristics:
o A service extension is described that allows a receiving Mail
Transport Agent (MTA) to signal the sending MTA that no soliciting
is in effect.
o A header field for the sender of the message is defined that
allows the sender to flag a message as conforming to a certain
class.
o Trace fields for intermediate MTAs are extended to allow the
intermediate MTA to signal that a message is in a certain class.
Allowing the sender of a message to tag a message as being, for
example, unsolicited commercial email with adult content, allows
"good" spammers to conform to legal content labelling requirements by
governmental authorities, license agreements with service providers,
or conventions imposed by "whitelist" services. For senders of mail
who choose not to abide by these conventions, the intermediate trace
fields defined here allow the destination MTAs to perform appropriate
dispositions on the received message.
This extension provides a simple mean for senders, MTAs, and
receivers to assert keywords. This extension does not deal with any
issues of authentication or consent.
2. The No-Soliciting SMTP Service Extension
Per [RFC2821], a "NO-SOLICITING" SMTP service extension is defined.
The service extension is declared during the initial "EHLO" SMTP
exchange. The extension has one optional parameter, consisting of
zero or more solicitation class keywords. Using the notation as
described in the Augmented BNF [RFC2234], the syntax is:
No-Soliciting-Service = "NO-SOLICITING"
[ SP Solicitation-keywords ]
As will be further described below, the "Solicitation-keywords"
construct is used to indicate which classes of messages are not
desired. A keyword that is presented during the initial "EHLO"
exchange applies to all messages exchanged in this session. As will
also be further described below, additional keywords may be specified
on a per-recipient basis as part of the response to a "RCPT TO"
command.
2.1. The EHLO Exchange
Keywords presented during the initial exchange indicate that no
soliciting in the named classes is in effect for all messages
delivered to this system. It is equivalent to the sign on the door
of an office building announcing a company-wide policy. For example:
R: <wait for connection on TCP port 25>
S: <open connection to server>
R: 220 trusted.example.com SMTP service ready
S: EHLO untrusted.example.com
R: 250-trusted.example.com says hello
R: 250-ENHANCEDSTATUSCODES
R: 250-NO-SOLICITING net.example:ADV
R: 250 SIZE 20480000
The "net.example:ADV" parameter to the "NO-SOLICITING" extension is
an example of a solicitation class keyword, the syntax of which is
described in the following section.
Historical Note:
A similar proposal was advanced in 1999 by John Levine and Paul
Hoffman. This proposal used the SMTP greeting banner to specify
that unsolicited bulk email is prohibited on a particular system
through the use of the "NO UCE" keyword [Levine]. As the authors
note, their proposal has the potential of overloading the
semantics of the greeting banner, which may also be used for other
purposes (see, e.g., [Malamud]).
2.2. Solicitation Class Keywords
The "NO-SOLICITING" service extension uses solicitation class
keywords to signify classes of solicitations that are not accepted.
Solicitation class keywords are separated by commas.
There is no default solicitation class keyword for the service. In
other words, the following example is a "no-op":
R : 250-NO-SOLICITING
While the above example is a "no-op" it is useful for an MTA that
wishes to pass along all messages, but would also like to pass along
"SOLICIT=" parameters on a message-by-message basis. The above
example invokes the use of the extension but does not signal any
restrictions by class of message.
The initial set of solicitation class keywords all begin with a
domain name with the labels reversed, followed by a colon. For
example, the domain name "example.com" could be used to form the
beginning of a solicitation class keyword of "com.example:". The
solicitation class keyword is then followed by an arbitrary set of
characters drawn from the following construct:
Solicitation-keywords = word
0*("," word)
; length of this string is limited
; to <= 1000 characters
word = ALPHA 0*(wordchar)
wordchar = ("." / "-" / "_" / ":" / ALPHA / DIGIT)
A solicitation class keyword MUST be less than 1000 characters. Note
however that a set of keywords used in the operations defined in this
document must also be less than 1000 characters. Implementors are
thus advised to keep their solicitation class keywords brief.
Any registrant of a domain name may define a solicitation class
keyword. Discovery of solicitation class keywords is outside the
scope of this document. However, those registrants defining keywords
are advised to place a definition of their solicitation class
keywords on a prominent URL under their control such that search
engines and other discovery mechanisms can find them.
While this document defines solicitation class keywords as beginning
with a reversed domain name followed by a colon (":"), future RFCs
may define additional mechanisms that do not conflict with this
naming scheme.
2.2.1. Note on Choice of Solicitation Class Keywords
This document does not specify which solicitation class keywords
shall or shall not be used on a particular message. The requirement
to use a particular keyword is a policy decision well outside the
scope of this document. It is expected that relevant policy bodies
(e.g., governments, ISPs, developers, or others) will specify
appropriate keywords, the definition of the meaning of those
keywords, and any other policy requirements, such as a requirement to
use or not use this extension in particular circumstances.
During discussions of this proposal, there were several suggestions
to do away with the solicitation class keywords altogether and
replace the mechanism with a simple boolean (e.g., "NO-SOLICITING
YES" or "ADV" or "UBE"). Under a boolean mechanism, this extension
would have to adopt a single definition of what "YES" or other label
means. By using the solicitation class keywords approach, the mail
infrastructure remains a neutral mechanism, allowing different
definitions to co-exist.
2.3. The MAIL FROM Command
"SOLICIT" is defined as a parameter for the "MAIL FROM" command. The
"SOLICIT" parameter is followed by an equal sign and a comma
separated list of solicitation class keywords. The syntax for this
parameter is:
Mail-From-Solicit-Parameter = "SOLICIT"
"=" Solicitation-keywords
; Solicitation-keywords, when used in MAIL FROM command
; MUST be identical to those in the Solicitation: header.
Note that white space is not permitted in this production.
As an informational message, the "550" or "250" replies to the "RCPT
TO" command may also contain the "SOLICIT" parameter. If a message
is being rejected due to a solicitation class keyword match,
implementations SHOULD echo which solicitation classes are in effect.
See Section 2.4 for more on error reporting.
The receiving system may decide on a per-message basis the
appropriate disposition of messages:
R: <wait for connection on TCP port 25>
S: <open connection to server>
R: 220 trusted.example.com SMTP service ready
S: EHLO untrusted.example.com
R: 250-trusted.example.com says hello
R: 250-NO-SOLICITING net.example:ADV
S: MAIL FROM:<save@example.com> SOLICIT=org.example:ADV:ADLT
S: RCPT TO:<coupon_clipper@moonlink.example.com>
R: 250 <coupon_clipper@moonlink.example.com>... Recipient ok
S: RCPT TO:<grumpy_old_boy@example.net>
R: 550 <grumpy_old_boy@example.net> SOLICIT=org.example:ADV:ADLT
In the previous example, the receiving MTA returned a "550" status
code, indicating that one message was being rejected. The
implementation also echoes back the currently set keywords for that
user on the "550" status message. The solicitation class keyword
which is echoed back is "org.example:ADV:ADLT" which illustrates how
this per-recipient solicitation class keyword has supplemented the
base "net.example:ADV" class declared in the "EHLO" exchange.
It is the responsibility of a receiving MTA to maintain a consistent
policy. If the receiving MTA will reject a message because of
solicitation class keywords, the MTA SHOULD declare those keywords
either in the initial "EHLO" exchange or on a per-recipient basis.
Likewise, a receiving MTA SHOULD NOT deliver a message where the
"Solicitation:" matches a solicitation class keyword that was
presented during the initial "EHLO" exchange or on a per-recipient
basis.
Developers should also note that the source of the solicitation class
keywords used in the "MAIL FROM" command MUST be the "Solicitation:"
header described in Section 2.5 and MUST NOT be supplemented by
additional solicitation class keywords derived from the "Received:"
header trace fields which are described in Section 2.6.
2.4. Error Reporting and Enhanced Mail Status Codes
If a session between two MTAs is using both the "NO-SOLICITING"
extension and the Enhanced Mail Status Codes as defined in [RFC3463]
and a message is rejected based on the presence of a "SOLICIT"
parameter, the correct error message to return will usually be
"5.7.1", defined as "the sender is not authorized to send to the
destination... (because) of per-host or per-recipient filtering."
Other codes, including temporary status codes, may be more
appropriate in some circumstances and developers should look to
[RFC3463] on this subject. An example of such a situation might be
the use of quotas or size restrictions on messages by class. An
implementation MAY impose limits such as message size restrictions
based on solicitation classes, and when such limits are exceed they
SHOULD be reported using whatever status code is appropriate for that
limit.
In all cases, an implementation SHOULD include a "Mail-From-Solicit-
Parameter" on a "550" or other reply that rejects message delivery.
The parameter SHOULD includes the solicitation class keyword(s) that
matched. In addition to the solicitation class keyword(s) that
matched, an implementation MAY include additional solicitation class
keywords that are in effect.
2.5. Solicitation Mail Header
Per [RFC2822], a new "Solicitation:" header field is defined which
contains one or more solicitation class keywords.
Solicitation-header = "Solicitation:" 1*SP Solicitation-keywords
An example of this header follows:
To: Coupon Clipper <coupon_clipper@moonlink.example.com>
From: Spam King <save@burntmail.example.com>
Solicitation: net.example:ADV,org.example:ADV:ADLT
Several proposals, particularly legal ones, have suggested requiring
the use of keywords in the "Subject:" header. While embedding
information in the "Subject:" header may provide visual cues to end
users, it does not provide a straightforward set of cues for computer
programs such as mail transfer agents. As with embedding a "no
solicitation" message in a greeting banner, this overloads the
semantics of the "Subject:" header. Of course, there is no reason
why both mechanisms can’t be used, and in any case the
"Solicitation:" header could be automatically inserted by the
sender’s Mail User Agent (MUA) based on the contents of the subject
line.
2.6. Insertion of Solicitation Keywords in Trace Fields
The "Solicitation:" mail header is only available to the sending
client. RFCs 2821 and 2822 are quite specific that intermediate MTAs
shall not change message headers, with the sole exception of the
"Received:" trace field. Since many current systems use an
intermediate relay to detect unsolicited mail, an addition to the
"Received:" header is described.
[RFC2821] documents the following productions for the "Received:"
header in a mail message:
; From RFC 2821
With = "WITH" FWS Protocol CFWS
Protocol = "ESMTP" / "SMTP" / Attdl-Protocol
Additionally, [RFC2822] defines a comment field as follows:
; From RFC 2822
comment = "(" *([FWS] ccontent) [FWS] ")"
ccontent = ctext / quoted-pair / comment
The "Mail-From-Solicit-Parameter" defined in Section 2.3 above is a
restricted form of ctext, yielding the following production:
With-Solicit = "WITH" FWS Protocol
"(" [FWS] comment [FWS] ")"
comment = "(" *([FWS] ccontent) [FWS] ")"
ccontent = ctext / quoted-pair /
comment / Mail-From-Solicit-Parameter
; The Mail-From-Solicit-Parameter
; is a restricted form of ctext
An example of a Received: header from a conforming MTA is as follows:
Received: by foo-mta.example.com with
ESMTP (SOLICIT=net.example:ADV,org.example:ADV:ADLT) ;
Sat, 9 Aug 2003 16:54:42 -0700 (PDT)
It should be noted that keywords presented in trace fields may not