Network Working Group C. Malamud
Internet-Draft October 10, 2003
Expires: April 9, 2004
A "No Soliciting" SMTP Service Extension
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (2003). All Rights Reserved.
Abstract
This note presents an extension to SMTP for an electronic mail
equivalent to the real-world "No Soliciting Sign." By itself, this
extension does little to stop unsolicited bulk electronic mail.
However, the extension gives policy makers in the real world a "hook"
on which to pass anti-spam laws.
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 [21].
Changes From Previous Draft
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Changes from draft-malamud-no-soliciting-00 to
draft-malamud-no-soliciting-01:
o The two service extensions previously proposed (
"SYSTEM-WIDE-NO-SOLICITING" and "PER-MESSAGE-NO-SOLICITING") have
been collapsed into a single "NO-SOLICITING" service extension See
Section 3.
o "PER-MESSAGE" has been changed to "PER-RECIPIENT" to more properly
express the operation of the extension. Section 3.3.
o A solicitation class keyword syntax is introduced to allow
different kinds of unsolicited mail to be considered.Section 3.2
o The "Solicitation:" header has been supplemented with an extended
"Received:" header syntax. See Section 3.5.
o A discussion of the use of Enhanced Mail Status Codes has been
included. See Section 3.7.
o A more extensive IANA Considerations section has been added,
including creation of a Solicitation Keywords registry. See
Section 6.
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Table of Contents
1. The Spam Pandemic . . . . . . . . . . . . . . . . . . . . . . 4
2. No Soliciting in the Real World . . . . . . . . . . . . . . . 6
3. The No-Soliciting SMTP Service Extension . . . . . . . . . . . 8
3.1 SYSTEM-WIDE . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.2 Solicitation Class Keywords . . . . . . . . . . . . . . . . . 8
3.3 PER-RECIPIENT . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4 Solicitation Mail Header . . . . . . . . . . . . . . . . . . . 10
3.5 Insertion of Solicitation Keywords in Trace Fields . . . . . . 11
3.6 Relay of Messages . . . . . . . . . . . . . . . . . . . . . . 12
3.7 Use of Enhanced Mail Status Codes . . . . . . . . . . . . . . 12
3.8 Recommendations for Developers and Administrators . . . . . . 13
4. Hooks for ISPs and Other Policy Makers . . . . . . . . . . . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6.1 The Mail Parameters Registry . . . . . . . . . . . . . . . . . 16
6.2 ESMTP-Solicitation Additional Protocol . . . . . . . . . . . . 16
6.3 The Solicitation Class Keywords Registry . . . . . . . . . . . 16
6.4 The Solicitation Mail Header . . . . . . . . . . . . . . . . . 17
7. Author's Acknowledgements . . . . . . . . . . . . . . . . . . 18
Informative References . . . . . . . . . . . . . . . . . . . . 19
Normative References . . . . . . . . . . . . . . . . . . . . . 21
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 21
Intellectual Property and Copyright Statements . . . . . . . . 22
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1. The Spam Pandemic
Unsolicited Unwanted Email (UUE), otherwise known as spam, has become
as one of the most pressing issues on the Internet. One oft-quoted
study estimated that spam will cost businesses $13 billion in
2003.[1] In April 2003, AOL reported that it had blocked 2.37 billion
pieces of UUE in a single day. [2] And, in a sure sign that UUE has
become of pressing concern, numerous politicians have begun to issue
pronouncements and prescriptions for fighting this epidemic.[3] [4]
A variety of mechanisms from the technical community have been
proposed and/or implemented to fight UUE:
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. [5]
o Blacklists are lists of known spammers or ISPs that allow spam.[6]
o Spam filters run client-side or server-side to filter out spam
based on whitelists, blacklists, and textual and header
analysis.[7]
o A large number of documents address the overall technical
considerations for the control of UUE [8], operational
considerations for SMTP agents[9], and various extensions to the
protocols to support UUE identification and filtering. [10] [11]
[12]
o Various proposals have been advanced for "do not spam" lists, akin
to the Federal Trade Commission's "Do Not Call" list for
telemarketers.[13]
Many of these proposals and services 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, whether or not
such a message is properly identified as belonging to that class.
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,
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disk space, and processing power.
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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." [14]
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 ..." [15]
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.[16]
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."[17] And, in Martin v. City of
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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."[18] Note that the public safety issue applies very much to
email, where viruses and can easily be delivered, in contrast to
telephone solicitations where public safety is not nearly as much an
issue.
This analysis is very U.S.-centric, which may be appropriate given
that the large majority of UUE appears to originate from U.S.
citizens. 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.
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3. The No-Soliciting SMTP Service Extension
Per RFC 2821,[22] a "NO-SOLICITING" SMTP service extension is
defined. The service extension is presented during the initial "EHLO"
SMTP exchange. The extension has one optional parameter and zero or
more solicitation class keywords. Using the notation as described in
the Augmented BNF[23], the syntax is:
No-Soliciting-Service = "NO-SOLICITING"
[ "SYSTEM-WIDE" / "PER-RECIPIENT" ]
0*( Solicitation-keywords )
3.1 SYSTEM-WIDE
"NO-SOLICITING SYSTEM-WIDE" indicates that no soliciting 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.
The parameter is presented during the initial exchange between sender
and receiver:
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 SYSTEM-WIDE
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 UCB" keyword.[19] 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., [20]).
3.2 Solicitation Class Keywords
The "NO-SOLICITING" service extension may accept additional
solicitation class keywords that signify a specific class of
solicitations that are not accepted. Keywords are separated by
commas and follow the "SYSTEM-WIDE" parameter.
Two classes are defined in this draft:
Keywords Description Reference
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--------- -------------------------------- ---------
MAPS-UBE Unsolicited Bulk Email http://mail-abuse.org/
ADV Unsolicited Commercial Email http://www.spamlaws.com/
ADV:ADLT Sexually Explicit Commercial Mail http://www.spamlaws.com/
MAPS-UBE is the standard advanced by the Mail Abuse Prevention System
(MAPS), which states:
An electronic message is "spam" IF: (1) the recipient's personal
identity and context are irrelevant because the message is equally
applicable to many other potential recipients; AND (2) the
recipient has not verifiably granted deliberate, explicit, and
still-revocable permission for it to be sent; AND (3) the
transmission and reception of the message appears to the recipient
to give a disproportionate benefit to the sender.
Numerous states have adopted the "ADV" and "ADV:ADLT" conventions.
We cite the spamlaws.com site as a reference because it provides an
excellent summary of the definitions and pointers to the relevant
statutes.
There is no default keyword for the service. In other words, the
following example is a "no-op":
R: 250-NO-SOLICITING SYSTEM-WIDE
Additional solicitation class keywords may be defined and registered
in the registry as specified in Section 6. Multiple solicitation
class keywords are separated by a comma to form a list:
Solicitation-keywords = 1Solicit-word 0*("," 1Solicit-word)
Solicit-word = [ "MAPS-UBE" / "ADV" / "ADV:ADLT"
/ x-word / registered-word ]
x-word = ["x-" / "X-"] 1*(wordchars)
registered-word = ALPHA 0*(wordchars)
; registered-word(s) are registered
; with the IANA
wordchars = 1*("-" / "_" / ":" / ALPHA / DIGIT)
3.3 PER-RECIPIENT
The "NO-SOLICITING PER-RECIPIENT" extension specifies that each "MAIL
FROM" command must identify if a message is a solicitation.
The presence of this extension is identified during the initial
greeting:
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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 PER-RECIPIENT
Additionally, "SOLICIT" is defined as a parameter for the "MAIL FROM"
command. The "SOLICIT" parameter is followed by an optional equal
sign and a comma separated list of solicitation class keywords.
The syntax for this parameter is:
Mail-From-Solicit-Parameter = "SOLICIT"
1( "=" Solicitation-keywords)
As an informational message, the 550 or 250 replies to the "RCPT TO"
command may also contain the "SOLICIT" parameter.
The receiving system may decide on a per-user basis the appropriate
disposition of messages:
S: MAIL FROM:<savebigbucks@hotmail.com> SOLICIT=ADV, MAPS-UBE
S: RCPT TO:<coupon_clipper@trusted.resource.org>
R: 250 <coupon_clipper@trusted.resource.org>... Recipient ok
S: RCPT TO:<grumpy_old_boy@trusted.resource.org>
R: 550 <grumpy_old_boy@trusted.resource.org>... SOLICIT=ADV:ADLT
3.4 Solicitation Mail Header
Per RFC 2822,[24] a new "Solicitation:" header field is defined which
contains one or more solicitation class keywords.
To: Coupon Clipper <coupon_clipper@trusted.resource.org>
From: Spam King <savebigbucks@hotmail.com>
Solicitation: ADV,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 would overload 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 based on the
contents of the subject line.
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3.5 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.
As a review, RFC 2821[22] documents the following productions for the
"Received:" header in a mail message:
Time-stamp-line = "Received:" FWS Stamp <CRLF>
Stamp = From-domain By-domain Opt-info ";" FWS date-time
; where "date-time" is as defined in [32]
; but the "obs-" forms, especially two-digit
; years, are prohibited in SMTP and MUST NOT be used.
From-domain = "FROM" FWS Extended-Domain CFWS
By-domain = "BY" FWS Extended-Domain CFWS
Extended-Domain = Domain /
( Domain FWS "(" TCP-info ")" ) /
( Address-literal FWS "(" TCP-info ")" )
TCP-info = Address-literal / ( Domain FWS Address-literal )
; Information derived by server from TCP connection
; not client EHLO.
Opt-info = [Via] [With] [ID] [For]
With = "WITH" FWS Protocol CFWS
Addtl-Link = Atom
; Additional standard names for links are registered with the
; Internet Assigned Numbers Authority (IANA). "Via" is
; primarily of value with non-Internet transports. SMTP
; servers SHOULD NOT use unregistered names.
Protocol = "ESMTP" / "SMTP" / Attdl-Protocol
Attdl-Protocol = Atom
; Additional standard names for protocols are registered with
; the Internet Assigned Numbers Authority (IANA). SMTP servers
; SHOULD NOT use unregistered names.
The appropriate location for solicitation information is the
"Attdl-Protocol" field, which is defined in this document as
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"ESMTP-Solicitation". The RFC 2821 productions are supplemented as
follows:
Protocol = "ESMTP" / "SMTP" / ESMTP-Solicitation / Attdl-Protocol
ESMTP-Solicitation = "ESMTP-Solicitation"
FWS 0*( Solicitation-keywords )
An example of a Received: header from a conforming MTA is as follows:
Received: by foo-mta.example.com with
ESMTP-Solicitation ADV,ADV:ADLT ; Sat, 9 Aug 2003
16:54:42 -0700 (PDT)
3.6 Relay of Messages
The "NO-SOLICITING SYSTEM-WIDE" service extension, if present,
applies to all messages handled by the receiving Message Transfer
Agent (MTA), including those messages intended to be relayed to
another system.
When relaying a message which was received via the SMTP protocol in
which the "SOLICIT" parameter was set on the "MAIL FROM" command, the
MTA MUST also set the "SOLICIT" parameter when delivering the message
to an SMTP server that supports this extension.
The "SOLICIT" parameter on a "MAIL FROM" command can be derived from
a variety of sources, including receipt of a message from a
conforming SMTP server. An SMTP server MAY, for operational reasons
as detailed in Section 7.7 of RFC 2821[22], set this parameter after
detecting the presence of the "Solicitation:" or extended "Received:"
message header field or by using other system-specific techniques.
Implementers should be aware that the "NO-SOLICITING" service
extension is not a guaranteed end-to-end service. Specifically,
intermediate relays that do not support this service may "lose" the
per-message parameters. However, any trace fields inserted during the
message transfer process will persist.
3.7 Use of 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 RFC
3463[25] and a message is rejected based on the presence of a
"SOLICIT" parameter, the correct error message to return is "5.6.0".
This error message is defined as being of class permanent failure
because it "is not likely to be resolved by resending the message in
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the current form. Some change to the message or the destination must
be made for successful delivery." It is defined as subject/detail
6.0 since the policy decision is based on message content and there
are not any detail fields that clearly fit this "error" in delivery.
If the "NO-SOLICITING" service extension is adopted, it is
recommended that a new detail code be added to subject 6 (i.e.,
"X.6.6 Message Not Accepted For Delivery").
3.8 Recommendations for Developers and Administrators
It is strongly recommended that any developers that implement the
"NO-SOLICITING" service extension SHOULD NOT enable the service as a
default. There are some indications that some policy makers may view
a default filtering in software as a prior restraint on commercial
speech. In other words, because the person installing the software
did not make an explicit choice to enable a certain type of
filtering, some might argue that such filtering was not desired.
Likewise, it is recommended that a system administrator installing
software SHOULD NOT enable "PER-RECIPIENT" filtering by default for a
user. Again, individual users should request the service.
The mechanism for an individual user to communicate their desire to
enable certain types of filtering is outside the scope of this
document.
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4. Hooks for ISPs and Other Policy Makers
This proposal is not meant to "solve" the UUE problem, but offers
some tools that can be used by policy makers, be they governments
defining laws or Internet Service Providers defining appropriate use
policies.
By providing a service-level extension to SMTP, this proposal
provides a simple mechanism that allows a system or ISP to put email
senders on notice that mail that is both bulk and unsolicited is not
wanted.
One common criticism of any technical or legal measures to prevent
UUE is that the global reach of the Internet makes any such measures
futile. Several points are worth noting:
1. First, anti-UUE complaints are often pursued through the
Appropriate Use Policy (AUP) in a service agreement between an
Internet Service Provider and an end user is accused of violating
the ISP's AUP. Assuming the Appropriate Use Policy is part of a
valid contract, conflicts of law do not exist in this case.
2. Disparity between laws of different jurisdictions is an age-old
problem and many mechanisms have evolved to solve these issues.
In the United States, conflicts of state laws are dealt with
through the courts and a well-established body of law.
3. On an international level, conflicts of law are dealt with
through international agreements, particularly trade agreements.
Thus, if the U.S. believes that UUE is a pressing policy issue,
it will bring the issue into a forum such as the World Trade
Organization, trading off a stronger agreement on spam for a more
liberal policy, for example, on the import of packaged meat
products.
4. Anectodal evidence suggests that much if not most UUE originates
from U.S. citizens. A policy "hook" in the SMTP architecture
will prove highly effective at a national level if not
universally effective on a global level.
In summary, no one proposal will solve all issues with unsolicited
unwanted email, but adding a mechanism at the SMTP service level
provides one more tool in that fight.
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5. Security Considerations
This proposal does not present additional security complications
beyond those already amply represented in the current architecture
for electronic mail.
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6. IANA Considerations
There are four IANA considerations presented in this draft:
1. Addition of the "NO-SOLICITING" service extension to the Mail
Parameters registry.
2. Addition of the "ESMTP-Solicitation" Additional Protocol
3. Creation of a Solicitation Class Keywords registry.
4. Creation of a "Solicitation:" mail header, which does not
currently raise any IANA considerations.
6.1 The Mail Parameters Registry
The IANA Mail Parameters registry documents SMTP service extensions.
The "NO-SOLICITATION" service extension would need to be added to
this registry as follows.
Keywords Description Reference
------------ -------------------------------- ---------
NO-SOLICITING Notification of no soliciting. [<this draft>]
The parameters subregistry would need to be modified as follows:
Service Ext EHLO Keyword Parameters Reference
----------- ------------ ----------- ---------
No Soliciting NO-SOLICITING SYSTEM-WIDE [<this draft>]
No Soliciting NO-SOLICITING PER-RECIPIENT [<this draft>]
6.2 ESMTP-Solicitation Additional Protocol
The Mail Parameters registry would need to be modified to list
"ESMTP-Solicitation" as a valid additional protocol for use in the
"Received:" header of a mail message.
6.3 The Solicitation Class Keywords Registry
A new registry (or a subregistry of Mail Parameters) would need to be
established for Solicitation Class Keywords. The registry would
contain the following fields:
1. Keyword name (e.g., "MAPS-UBE").
2. Keyword description (e.g., "Unsolicited Bulk Email").
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3. Keyword reference (e.g., "<this draft>").
Per the policies outlined in RFC 2434[26], it is recommended that the
IESG appoint a Designated Expert to administer this registry.
Authority for solicitation class keywords in this registry will come
in some cases from published RFCs, but in other cases will come from
applicable laws or regulations. It is recommended that any non-RFC
derived solicitation class keywords be documented in future
informational RFCs to provide a consistent set of references.
6.4 The Solicitation Mail Header
There is currently no registry defined for mail headers. If such a
registry were to exist, the "Solicitation:" header field would need
to be added to it.
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7. Author's Acknowledgements
The author would like to thank Rebecca Malamud for many discussions
and ideas that led to this proposal and to John C. Klensin and
Marshall T. Rose for their extensive input on how it could be
properly implemented in SMTP. Dave Crocker, Paul Hoffman, John
Levine, Keith Moore, Paul Vixie, and Pindar Wong kindly provided
reviews of the draft and/or suggestions for improvement. Information
about soliciting outside the U.S. was received from Rob Blokzijl, Jon
Crowcroft, Christian Huitema, Geoff Huston, and Pindar Wong. As
always, all errors, omissions, generalizations, and simplifications
(EOGS) are the responsibility of the author.
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Informative References
[1] Associated Press, "Study: Spam costs businesses $13 billion",
January 2003.
[2] CNET News.Com, "AOL touts spam-fighting prowess", April 2003.
[3] Charles, C., "Schumer, Christian Coalition Team Up to Crack
Down on Email Spam Pornography", June 2003.
[4] Federal Trade Commission, "Federal, State, Local Law Enforcers
Target Deceptive Spam and Internet Scams", November 2002.
[5] Habeas, Inc., "Habeas Compliant Message", April 2003.
[6] Spamhaus.Org, "Register of Known Spam Operations".
[7] Mason, J., "Spamassassin - Mail Filter to Identify Spam Using
Text Analysis", Version 2.55, May 2003.
[8] Crocker, D., "Technical Considerations for Spam Control
Mechanisms", draft-crocker-spam-techconsider-01 (work in
progress), May 2003.
[9] Lindberg, G., "Anti-Spam Recommendations for SMTP MTAs", BCP
30, RFC 2505, February 1999.
[10] Danisch, H., "A DNS RR for simple SMTP sender authentication",
draft-danisch-dns-rr-smtp-02 (work in progress), June 2003.
[11] Daboo, C., "SIEVE Spamtest and Virustest Extensions",
draft-daboo-sieve-spamtest-03 (work in progress), April 2003.
[12] Crouzet, B., "Authenticated Mail Transfer Protocol",
draft-crouzet-amtp-00 (work in progress), June 2003.
[13] Federal Trade Commission, "Telemarketing Sales Rule", Federal
Register Vol. 68, No. 19, January 2003.
[14] The Town of West Newbury, Massachusetts, "Soliciting/Canvassing
By-Law", Chapter 18 Section 10, March 2002.
[15] U.S. Supreme Court, "Watchtower Bible & Tract Society of New
York, Inc., et al. v. Village of Stratton et al.", 122 S.Ct.
2080 (2002), June 2002.
[16] U.S. Supreme Court, "Perry Education Association v. Perry Local
Educators' Association", 460 U.S. 37 (1983), February 1983.
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[17] U.S. Supreme Court, "Cantwell v. State of Connecticut", 310
U.S. 296 (1940), May 1940.
[18] U.S. Supreme Court, "Martin v. City of Struthers, Ohio", 319
U.S. 141 (1943), May 1943.
[19] Levine, J. and P. Hoffman, "Anti-UBE and Anti-UCE Keywords in
SMTP Banners", Revision 1.1, March 1999.
[20] Malamud, C., "An Internet Prayer Wheel", Mappa.Mundi Magazine,
August 1999.
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Normative References
[21] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[22] Klensin, J., "Simple Mail Transfer Protocol", RFC 2821, April
2001.
[23] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[24] Resnick, P., "Internet Message Format", RFC 2822, April 2001.
[25] Vaudreuil, G., "Enhanced Mail System Status Codes", RFC 3463,
January 2003.
[26] Narten, T. and H. Alvestrand, "Guidelines for Writing an IANA
Considerations Section in RFCs", BCP 26, RFC 2434, October
1998.
Author's Address
Carl Malamud
PO Box 300
Sixes, OR 97476
US
EMail: carl@media.org
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