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Internet-Draft                                                  K. Moore
Expires: 6 February 2004                         University of Tennessee
                                                           6 August 2003


       Recommendations for Automatic Responses to Electronic Mail

                   draft-moore-auto-email-response-02


Status of this Memo

This document is an Internet-Draft and is subject to all provisions of
Section 10 of RFC2026.

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Abstract

This memo makes recommendations for software that automatically responds
to incoming electronic mail messages, including "out of the office" or
"vacation" response generators, mail filtering software, email-based
information services, and other automatic responders.  The purpose of
these recommendations is to discourage undesirable behavior which is
caused or aggravated by such software, to encourage uniform behavior
(where appropriate) among automatic mail responders, and to clear up
some sources of confusion among implementors of automatic email
responders.




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Intended status: Once it appears that this document has received
sufficient review, comment, and community support, the author intends to
submitted it as an individual submission for Proposed Standard status.
Proposed Standard seems more appropriate than BCP because this document
describes protocols more than operational practices.

1. Introduction

Many programs which automatically respond to email are currently in use.
Although these programs vary widely in their function, several problems
with this class of programs have been observed, including: significant
numbers of useless or unwanted response and responses sent to
inappropriate addresses, and occasional incidences of mail loops or
"sorcerer's apprentice" mode.  This memo recommends behavior for
programs that automatically respond to electronic mail in order to
reduce the number of problems caused by such programs.

(Note: the term "sorcerer's apprentice mode" is defined as a bug in a
protocol where, under some circumstances, the receipt of a message
causes multiple messages to be sent, each of which, when received,
triggers the same bug.) (From [I1])

1.1 Types of automatic responses

There are several different types of automatic responses.  At least two
types of automatic responses have been defined in IETF standards -
Delivery Status Notifications [I2] which are intended to report the
status of a message delivery by the message transport system, and
Message Disposition Notifications [I3] which are intended to report of
the disposition of a message after it reaches a recipient's mailbox.
These responses are defined elsewhere and are generally not within the
purview of this document, except that this document recommends specific
cases where they should or should not be used.

Other types of automatic response in common use include:

-    "Out of office" or "vacation" notices, which are intended to inform
     the sender of a message that the message is unlikely to be read, or
     acted on, for some amount of time;

-    "Change of address" notices, intended to inform the sender of a
     message that the recipient address he used is obsolete and that a
     different address should be used instead (whether or not the
     subject message was forwarded to the current address).

-    "Challenges", which require the sender of a message to demonstrate
     some measure of intelligence and/or willingness to agree to some
     conditions before the subject message will be delivered to the



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     recipient (often to minimize the effect of "spam" or viruses on the
     recipient).

-    Email-based information services, which accept requests (presumably
     from humans) via email, provide some service, and issue responses
     via email also.  (Mailing lists which accept subscription requests
     via email fall into this category);

-    Information services similar to those mentioned above except that
     they are intended to accept messages from other programs;

-    Various kinds of mail filters (including "virus scanners") which
     act on behalf of a recipient to alter the content of messages
     before forwarding them to that recipient, and issue responses in
     the event a message is altered; and

Recognizing the wide variety of response types in use, these
recommendations distinguish between several classes of automatic
responders according to the party or service on whose behalf the
responder acts:

-    "Service Responders" exist to provide access to some service via
     email requests and responses.  These are permanently associated
     with one or more email addresses, and when sending to such an
     address the sender presumably expects an automatic response.  An
     email-based file retrieval service is an example of a Service
     Responder.  A calendar service that allowed appointment requests to
     be made via email, and which responded to such requests, would be
     another example of a Service Responder.

-    "Personal Responders" exist to make automatic responses on behalf
     of a single recipient address, in advance of, or in lieu of, that
     recipient reading the message.  These responders operate according
     to criteria specified on a per-recipient basis.  The UNIX
     "vacation" program is an example of a Personal Responder.  A
     responder that accepts mail sent to a single address, attempts to
     analyze and classify the contents, and then issues a response which
     is dependent on that classification, is also a Personal Responder.

-    "Group Responders" exist to make automatic responses on behalf of
     any of a significant set of recipient addresses (say, every
     recipient in a particular DNS domain), in advance of, or in lieu
     of, a response from the actual recipient.  Group Responders are
     similar to Personal Responders except that in the case of a Group
     Responder the criteria for responding are not set on a per-
     recipient basis.  A "virus scanner" program that filtered all mail
     sent to any recipient on a particular server, and sent responses
     when a message was rejected or delivered in an altered form, might



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     be an example of a Group Responder.

Appropriate behavior for a responder varies from one class to another.
A behavior which might be appropriate from a Service Responder (where
the sender is expecting an automatic response) might not be appropriate
from a Personal Responder.  For example, a Service Responder might send
a very long response to a request, or one that is not in a human-
readable format, according to the needs of that service.  However a
Personal Responder should assume that a human being is reading the
response and send only brief responses in plain text.

1.2. Notation and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in [N1].

The term "subject message" is used to refer to a message which causes a
response to be sent.

The term "response" refers to a message that is automatically issued on
receipt of a subject message by a responder.

A "responder" is a process that automatically responds to subject
messages under some well-defined set of conditions.

Unless specified otherwise, the term "recipient" refers to the email
addresses to which a subject message was delivered (rather than, for
instance, the address to which the response was sent).  A "recipient"
address might be permanently associated with a responder, or it might be
the address of a human being whose mail is, under some conditions,
answered by a responder.

2. When (not) to send automatic responses

An automatic responder MUST NOT send a response for every message
received.  In practice there are always reasons to refuse to respond to
some kinds of received messages, e.g. for loop prevention, to avoid
responding to "spam", to avoid being used as a means to launder or
amplify abusive messages, and to thwart denial-of-service attacks
against the responder.  The criteria for deciding whether to respond
will differ from one responder to another, according to the responder's
purpose.  In general, care should be taken to avoid sending useless or
redundant responses, and to avoid contributing to mail loops or
facilitating denial-of-service attacks.

Here are some broad guidelines:




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-    Automatic responses SHOULD NOT be issued in response to any message
     which contains an Auto-Submitted header field (see below), where
     that field has any value other than "no".

-    Personal and Group responses that are intended to notify the sender
     of a message of the recipient's inability to read or reply to the
     message (e.g. "away from my mail" or "too busy" notifications)
     SHOULD NOT issue the same response to the same sender more than
     once within a period of several days, even though that sender may
     have sent multiple messages.  A 7-day period is RECOMMENDED as a
     default.

-    Personal and Group responses whose purpose is to notify the sender
     of a message of a temporary absence of the recipient (e.g.
     "vacation" and "out of the office" notices) SHOULD NOT be issued
     unless a valid address for the recipient is explicitly included in
     a recipient (e.g. To, CC, or Bcc) field of the subject message.
     Since a recipient may have multiple addresses forwarded to the same
     mailbox, recipients SHOULD be able to specify a set of addresses to
     the responder which it will recognize as valid for that recipient.

-    Responders SHOULD NOT generate any response for which the
     destination of that response would be a null address (e.g. an
     address for which SMTP MAIL FROM or Return-Path is <>), since the
     response would not be delivered to a useful destination.
     Responders MAY refuse to generate responses for addresses commonly
     used as return addresses by responders - e.g. those with local-
     parts matching "owner-*", "*-request", "MAILER-DAEMON", etc.
     Responders are encouraged to check the destination address for
     validity before generating the response, to avoid generating
     responses that cannot be delivered or are unlikely to be useful.

-    In order to avoid responding to spam and to certain kinds of
     attacks, automatic responses from Service Responders SHOULD NOT be
     sent for extremely malformed requests.  This may include checking
     that the subject message has a content-type and content appropriate
     to that service.

-    Because the vast majority of email is unauthenticated, and return
     addresses are easily forged, in order to avoid being used as a
     means of denial-of-service attacks (i.e. to flood mailboxes with
     unwanted content) Service Responders SHOULD NOT return large
     responses (say, more than a few kilobytes) without specific
     knowledge that the request was actually authorized by the party
     associated with the address to which the response will be sent.
     Similarly, Service Responders SHOULD NOT cause unwanted side-
     effects (such as subscribing the sender to a mailing list) without
     reasonable assurance that the request was authorized by the



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     affected party.

     NOTE: Since each responder has a different purpose and a different
     set of potential threats to which it might be subjected, whether
     any particular means of authentication is appropriate for a
     particular responder is not in scope for this document.

-    A responder MAY refuse to send a response to a subject message
     which contains any header or content which makes it appear to the
     responder that a response would not be appropriate.  For instance,
     if the subject message contained a Precedence header field [I4]
     with a value of "list" the responder might guess that the traffic
     had arrived from a mailing list, and would not respond if the
     response were only intended for personal messages.  For similar
     reasons, a responder MAY ignore any subject message with a List-*
     field [I5].  (Because Precedence is not a standard header field,
     and its use and interpretation vary widely in the wild, no
     particular responder behavior in the presence of Precedence is
     recommended by this specification.)

3. Format of automatic responses

The following sections specify details of the contents of automatic
responses, including the header of the response message, the content of
the response, and the envelope in which the response is transmitted to
the email transport system.

3.1 Message header

The fields in the message header should be set as follows:

3.1.1 From field

In correspondence between humans, the From field serves multiple
purposes: It identifies the author of the message (or in some cases, the
party or parties on whose behalf the message was sent), and it is the
default destination of replies from humans.  Also, unfortunately some
mail systems still send nondelivery reports and other kinds of automatic
responses to the From address.

For automatic responses, the role of the From field in determining the
destination of replies to the response from humans is less significant,
because in most cases it is not useful or appropriate for a human (or
anyone) to reply to an automatic response.  One exception is when there
is some problem with the response; it should be possible to provide
feedback to the person operating the responder.





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So in most cases the From address in an automatic response needs to be
chosen according to the following criteria:

-    To provide an indication of the party or agent on whose behalf the
     response was sent,

-    To provide an address to which a recipient of an inappropriate
     response can request that the situation be corrected, and

-    To diminish the potential for mail loops.

The following behavior is thus recommended:

-    For responses sent by Service Responders, the From field SHOULD
     contain an address which can be used to reach the (human)
     maintainer of that service.  The human-readable portion of the From
     field (the phrase preceding the address) SHOULD contain a name or
     description of the service to identify the service to humans.

-    For responses sent by Personal Responders, the From field SHOULD
     contain the name of the recipient and an address chosen by the
     recipient to be recognizable to correspondents. Often this will be
     the same address that was used to send mail to that recipient.

     In the case of a recipient having multiple mail addresses forwarded
     to the same mailbox (and responder), a Personal Responder MAY use
     heuristics to guess, based on the information available in various
     message header fields, which of several addresses for that
     recipient the sender is likely to have used, and use that address
     in the From field of the response.  However it MUST be possible for
     a recipient on whose behalf the responder is to explicitly specify
     the human-readable name and address to be used in the From header
     fields of responses.

     Note: Due to privacy reasons it may be inappropriate for responders
     to disclose an address that is derived, say, from the recipient's
     login information (e.g. POP or IMAP user name or account name on a
     multiuser computer) or which discloses the specific name of the
     computer where the response was generated.  Furthermore these do
     not necessarily produce a valid public email address for the
     recipient.  For this reason the From field of a Personal Response
     MUST be settable by the recipient on whose behalf the responder is
     acting.

-    For Group Responders, the From address SHOULD contain an email
     address which could be used to reach the maintainer of that Group
     Responder.  Use of the Postmaster address for this purpose is NOT
     RECOMMENDED.



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     The human-readable portion of the From address (the "phrase" before
     the address, see [N2], section 3.2.6) SHOULD contain an indication
     of the function performed by the Group Responder and on whose
     behalf it operates (e.g. "Example Agency virus filter")

3.1.2 Reply-To field

If a reply is expected by the responder, the Reply-To field of the
response SHOULD be set to the address at which the reply is expected,
even if this is the address of the same or another responder.
Responders which request replies to be sent to responders MUST prevent
mail loops and sorcerer's apprentice mode.  Note that since (according
to the previous section) the From field of the response SHOULD contain
the address of a human, if the Reply-To field of the response is used to
direct replies to a responder it will not be the same as the address in
the From field.

Discussion: this assumes that the human recipient's user agent will
normally send replies to the Reply-To address (if present), as
recommended by [I6] since 1982, but that it is still possible for a
recipient to reply to the From address if he or she finds it useful to
do so.  This is consistent with the intended use of these fields in [I6]
and [N2].

3.1.3 To field

The To header field SHOULD indicate the recipient of the response.  In
general there SHOULD only be one recipient of any automatic response.
This minimizes the potential for sorcerer's apprentice mode and denial-
of-service attacks.

3.1.4 Date field

The Date header field SHOULD indicate the date and time at which the
response was generated.   This MUST NOT be taken as any indication of
the delivery date of the subject message, nor of the time at which the
response was sent.

3.1.5 Subject field

The Subject field SHOULD contain a brief indication that the message is
an automatic response, followed by contents of the Subject field (or a
portion thereof) from the subject message.  The prefix "Auto:" MAY be
used as such an indication.  If used, this prefix SHOULD be followed by
an ASCII SPACE character (0x20).

NOTE: Just as the (Latin-derived) prefix "Re:" that is commonly used to
indicate human-generated responses is sometimes translated to other



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languages by mail readers, or otherwise interpreted by mail readers as
indication that the message is a reply, so the (Greek) prefix "Auto:"
may also be translated or used as a generic indication that the message
is an automatic response.  However the "Auto:" indication is intended
only as an aid to humans in processing the message.  Mail processing
software SHOULD NOT assume that the presence of "Auto:" at the beginning
of a Subject field is an indication that the message was automatically
submitted.

Note that the Subject field of the subject message may contain encoded-
words formatted according to [N3], and such text MAY be included in the
Subject field of a response.  In generating responses containing such
fields there is rarely a need to decode and re-encode such text.  It is
usually sufficient to leave those encoded-words as they were in the
subject message, merely prepending "Auto: " or other indication.
However, it is still necessary to ensure that no line in the resulting
Subject field that contains an encoded-word is greater than 76 ASCII
characters in length (this refers to the encoded form, not the number of
characters in the text being encoded).  Also, if the responder truncates
the Subject from the subject message it is necessary to avoid truncating
Subject text in the middle of an encoded-word.

3.1.6 In-Reply-To and References fields

The In-Reply-To and References fields SHOULD be provided in the header
of the response message if there was a Message-ID field in the subject
message, according to the rules in [N2] section 3.6.4.

3.1.7 Auto-Submitted field

The Auto-Submitted field, with a value of "auto-replied", SHOULD be
included in the message header of any automatic response.  See section
5.

3.1.8 Precedence field

A response MAY include a Precedence field [I4] in order to discourage
responses from some kinds of responders which predate this
specification.  The field-body of the Precedence field MAY consist of
the text "junk", "list", "bulk", or other text deemed appropriate by the
responder.  Because the Precedence field is non-standard and its
interpretation varies widely, the use of Precedence is not specifically
recommended by this specification, nor does this specification recommend
any particular value for that field.







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3.2 Message content

In general, messages sent by Personal or Group Responders SHOULD be
brief, and in text/plain format.  A multipart/alternative construct MAY
be used to communicate responses in multiple languages, especially if in
doing so it is desirable to use multiple charsets.

Response messages SHOULD NOT include significant content from the
subject message. In particular, Personal and Group responses SHOULD NOT
contain non-text content from the subject message, and they SHOULD NOT
include attachments from the subject message.  Neither of these
conditions applies to responders that specifically exist for the purpose
of altering or translating content sent to them (for instance, a
FORTRAN-to-C translator); however, such responders MUST employ measures
to being used as a means of laundering or forwarding undesirable
content, such as spam or viruses.

Note that when text from the Subject or other fields from the header of
the subject message is included in the body of the response, it is
necessary to decode any encoded-words that appeared in those fields
before including in the message body, and to use an appropriate content-
type, charset, and content-transfer-encoding.  In some cases it may be
necessary to transliterate text from the charset(s) used in the header
of the subject message, to the charset(s) used in the body of the
response.  (It is much easier to implement a responder if text from the
header of the subject message never needs to appear in the body of the
response.)

3.2.1 Use of DSNs and MDNs instead of this specification

In general, it is appropriate to use Delivery Status Notifications
(DSNs) for responses that are generated by the mail transport system as
a result of attempts to relay, forward, or deliver mail, and only when
the purpose of that response is to provide the sender of the subject
message with information about the status of that mail delivery.  For
instance, a "virus scanner" which is activated by a mail delivery
process to filter harmful content prior to delivery, could return a DSN
with the Action field set to "failed" with a Status code of 5.7.1
(Delivery not authorized, message refused) if the entire message was not
delivered due to security reasons; or it could return a DSN with the
Action field set to "relayed" or "delivered" (as appropriate) with a
Status code set to 2.6.4 (conversion with loss performed) if the message
was relayed or delivered with the presumably harmful content removed.
The DSN specification [I7], rather than this document, governs the
generation and format of DSNs.

Similarly, it is appropriate to use Message Disposition Notifications
(MDNs) only for responses generated on the recipient's behalf, which are



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generated on or after delivery to a recipient's mailbox, and for which
the purpose of the response is to indicate the disposition of the
message.  The MDN specification [I8], rather than this document, governs
the generation and format of MDNs.

This document is not intended to alter either the DSN or MDN
specifications.  Responses that fit within the criteria of DSN or MDN,
as defined by the respective specifications, should be generated
according to the DSN or MDN specification rather than this document.
Responses which do not fit one of these sets of criteria should be
generated according to this document.

3.3 Message envelope

The SMTP MAIL FROM address, or other envelope return address used to
send the message, SHOULD be chosen in such a way as to make mail loops
unlikely.  A loop might occur, for instance, if both sender and
recipient of a message each have automatic responders - the recipient's
responder sends mail to the sender's responder, which sends mail back to
the recipient's responder.

The primary purpose of the MAIL FROM address is to serve as the
destination for delivery status messages and other automatic responses.
Since in most cases it is not appropriate to respond to an automatic
response, and the responder is not interested in delivery status
messages, a MAIL FROM address of <> MAY be used for this purpose.  A
MAIL FROM address which is specifically chosen for the purpose of
sending automatic responses, and which will not automatically respond to
any message sent to it, MAY be used instead of <>.

The RCPT TO address should be the address of the intended recipient of
the response.  It is RECOMMENDED that the NOTIFY=NEVER parameter of the
RCPT command be specified if the SMTP server supports the DSN option
[N4].

4. Where to send automatic responses (and where not to send them)

In general, automatic responses SHOULD be sent to the Return-Path field
if generated after delivery.  If the response is generated prior to
delivery, the response SHOULD be sent to the reverse-path from the SMTP
MAIL FROM command, or (in a non-SMTP system) to the envelope return
address which serves as the destination for nondelivery reports.

If the response is to be generated after delivery, and there is no
Return-Path field in the subject message, there is an implementation
error in the SMTP server that delivered the message, or that SMTP server
is improperly configured.  A Personal or Group responder SHOULD NOT
deliver a response to any address other than that in the Return-Path



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field, even if the Return-Path field is missing.  It is better to fix
the problem with the mail delivery system than to rely on heuristics to
guess the appropriate destination of the response.  Such heuristics have
been known to cause problems in the past.

A Service Responder MAY deliver the response to the address from the
>From field, or to another address from the request payload, provided
this behavior is precisely defined in the specification for that
service.  Services responders SHOULD NOT use the Reply-To field for this
purpose.

The Reply-To field SHOULD NOT be used as the destination for automatic
responses from Personal or Group Responders.  In general, this field is
set by a human sender based on his/her anticipation of how human
recipients will respond to the specific content of that message. For
instance, a human sender may use Reply-To to request that replies be
sent to an entire mailing list.  Even for replies from humans, there are
cases where it is not appropriate to respond to the Reply-To address,
especially if the sender has asked that replies be sent to a group
and/or mailing list.  Since a Personal or Group Responder operates on
behalf of a human recipient, it is safer to assume that any Reply-To
field present in the message was set by a human sender on the assumption
that any reply would come from a human who had some understanding of the
roles of the sender and other recipients.  An automatic responder lacks
the information necessary to understand those roles.  Sending automatic
responses to Reply-To addresses can thus result in a large number of
people receiving a useless or unwanted message; it can also contribute
to mail loops.

Use of the From field as the destination for automatic responses has
some of the same problems as use of Reply-To.  In particular, the From
field may list multiple addresses, while automatic responses should only
be sent to a single address.  In general, the From and Reply-To
addresses are used in a variety of ways according to differing
circumstances, and for this reason Personal or Group Responders cannot
reliably assume that an address in the From or Reply-To field is an
appropriate destination for the response.  For these reasons the From
field SHOULD NOT be used as a destination for automatic responses.

Similarly, the Sender field SHOULD NOT be used as the destination for
automatic responses.  This field is intended only to identify the person
or entity that sent the message, and is not required to contain an
address that is valid for replies.

The Return-Path address is really the only one from the message header
that can be expected, as a matter of protocol, to be suitable for
automatic responses that were not anticipated by the sender.




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5. The Auto-Submitted header field

The purpose of the Auto-Submitted header field is to indicate that the
message was originated by an automatic process, or an automatic
responder, rather than by a human; and to facilitate automatic filtering
of messages from signal paths for which automatically generated messages
and automatic responses are not desirable.

5.1 Syntax

The syntax of Auto-Submitted is as follows, using the ABNF notation of
[N5]:

auto-submitted-field     = "Auto-Submitted:" CFWS
                           auto-submitted [CFWS] CRLF

auto-submitted           = ( "no" / "auto-generated" /
                           "auto-replied" / extension )
                           opt-parameter-list

extension                = token

opt-parameter-list       = *( [CFWS] ";" [CFWS] LWSP parameter )


The symbols "token", and "parameter" are as defined in [N6].

5.2 Semantics

The Auto-Submitted header field SHOULD NOT be supplied for messages that
were manually submitted by a human.  (However, user agents that allow
senders to specify arbitrary fields SHOULD NOT prevent humans from
setting the Auto-Submitted field, because it is sometimes useful for
testing.)

The auto-generated keyword:

-    SHOULD be used on messages generated by automatic (often periodic)
     processes (such as UNIX "cron jobs"),

-    MUST NOT be used on manually generated messages,

-    MUST NOT be used on a message issued in direct response to another
     message.

The auto-replied keyword:





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-    SHOULD be used on messages sent in direct response to another
     message,

-    MUST NOT be used on manually-generated messages,

-    MUST NOT be used on messages generated by automatic or periodic
     processes.

The "no" keyword may be used to explicitly indicate that a message was
originated by a human.

Extension keywords may be defined in the future, though it seems
unlikely.  The syntax and semantics of such keywords must be published
as RFCs and approved using the IETF Consensus process [N7].  Keywords
beginning with "x-" are reserved for experiments and use among
consenting parties.  Recipients of messages containing an Auto-Submitted
field with any keyword other than "no" MAY assume that the message was
not manually submitted by a human.

Optional parameters may also be defined by an IETF Consensus process.
The syntax of optional parameters is given here to allow for future
definition should they be needed.  Implementations of Auto-Submitted
conforming to this specification MUST NOT fail to recognize an
Auto-Submitted field and keyword that contains syntactically valid
optional parameters, but such implementations MAY ignore those
parameters if they are present.  Parameter names beginning with "x-" are
reserved for experiments and use among consenting parties.

The "comment" syntactical construct from [N2] can be used to indicate a
reason why this message was automatically submitted.

6. Security Considerations

Automatic responders introduce the potential for several kinds of
attack, including:

-    Use of such responders to relay harmful or abusive content (worms,
     viruses, spam, and spymail) for the purpose of wider distribution
     of the content or masking the source of such content;

-    Use of such responders to mount denial-of-service attacks by using
     responders to relay messages to large numbers of addresses, or to
     flood individual mailboxes with a large amount of unwanted content,
     or both;

-    Deliberate or accidental use of such responders to construct mail
     loops or "sorcerer's apprentice mode", thus taxing the resources of
     the mail transport system;



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-    In addition, the responder itself may be subject to attack by
     sending it large numbers of requests.

This document attempts to reduce the vulnerability of responders to such
attack, in particular by

-    Recommending that responders not relay significant content from the
     subject message (thus minimizing the potential for use of
     responders to launder or amplify attacker-chosen content)

-    Recommending that responders clearly mark responses with the
     "Auto-Submitted: auto-replied" header field to distinguish them
     from messages originated by humans (in part, to minimize the
     potential for loops and denial-of-service attacks),

-    Recommending that Personal and Group Responders limit the number of
     responses sent to any individual per period of time (also limiting
     the potential damage caused by loops),

-    Recommending that responders respond to at most one address per
     incoming message (to minimize the potential for deliberate or
     accidental denial-of-service via "multiplication" or sorcerer's
     apprentice mode),

-    Recommending that responses from Personal and Group Responders
     should be brief and in plain text format (to minimize the potential
     for mail responders to be used as mechanisms for transmitting
     harmful content and/or disguising the source of harmful content).

However, because email addresses are easily forged, attacks are still
possible for any email responder which does not limit access and require
authentication before issuing a response.  The above measures attempt to
limit the damage which can be done, but they cannot entirely prevent
attacks.

This section describes vulnerabilities inherent in automatically
responding to mail.  Other vulnerabilities are associated with some
mail-based services which automatically respond to email messages, but
these are not caused by the fact that the server automatically responds
to incoming messages.  In general, any network-based service (including
those accessed by email) needs to provide security that is sufficient to
prevent the service from being used as a means to inappropriately or
destructively access the resources that are accessible by the service.

7. IANA Considerations

Section 5 of this document defines two new extension mechanisms - new
keywords for the Auto-Submitted header field, and new optional



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parameters for the Auto-Submitted field.  If at any point in the future
new keywords or parameters are approved (through an IETF Consensus
process) it may be appropriate for IANA to create a registry of such
keywords or parameters.

8. Acknowledgments

In the mid-1990s Jeroen Houttuin of TERENA authored a series of
internet-drafts on "Behavior of Mail Based Servers", and in particular,
one document on "Answering Servers" [I9].  While these documents were
(to this author's knowledge) never formally published, they provided the
first well-reasoned argument (known to this author) as to the best way
for such servers to interface with email systems and protocols.

The idea for the Auto-Submitted field comes from the X.400/MHS mail
system [I10].  [I11] defined an "Autosubmitted" field for use when
gatewaying between X.400 and Internet mail.  Jacob Palme wrote an
internet-draft [I12] defining use of the "Auto-Submitted" field for
Internet mail, which made it through Last Call without significant
objections, but got stalled in an attempt to resolve non-substantial
objections.  The definition of Auto-Submitted in this document is
derived (i.e. slightly simplified) from the one in that document, with
some text stolen outright.

Thanks are also due to those who contributed suggestions to this
document: (so far) Russ Allbery, Ned Freed, Lawrence Greenfield, Arnt
Gulbrandsen, Eric Hall, Tony Hansen, Dan Kohn, Lyndon Nerenberg, Florian
Weimer, and Dan Wing.

9. Author's Address

Keith Moore
Innovative Computing Laboratory
University of Tennessee, Knoxville
1122 Volunteer Blvd, #203
Knoxville, TN 37996-3450

moore@cs.utk.edu


10. Normative References

[N1]   Bradner, S. Key words for use in RFCs to Indicate Requirement
       Levels.  RFC 2119, March 1997.

[N2]   Resnick, P. (ed.) Internet Message Format.  RFC 2822, April 2001.





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[N3]   Moore, K.  MIME (Multipurpose Internet Mail Extensions) Part
       Three: Message Header Extensions for Non-ASCII Text.  RFC 2047,
       November 1996.

[N4]   Moore, K.  SMTP Service Extension for Delivery Status
       Notifications.  RFC 1891, January 1996.

[N5]   Crocker, D. (ed.), Overell, P. Augmented BNF for Syntax
       Specifications: ABNF. RFC 2234, November 1997.

[N6]   Freed, N. Borenstein, N.  Multipurpose Internet Mail Extensions
       (MIME) Part One: Format of Internet Message Bodies.  RFC 2045,
       November 1996.

[N7]   Narten, T., Alvestrand, H.  Guidelines for Writing an IANA
       Considerations Section in RFCs.  RFC 2434, October 1998.

11. Non-Normative (Informative) References

[I1]   "Sorcerer's apprentice mode", originally from the Jargon file
       once maintained at MIT-AI and SAIL; now collected at various
       places on the net.  See e.g. http://www.jargon.net/

[I2]   Moore, K. Vaudreuil, G.  An Extensible Message Format for
       Delivery Status Notifications.  RFC 1894, January 1996.

[I3]   Fajman, R.  An Extensible Message Format for Message Disposition
       Notifications. RFC 2298, March 1998.

[I4]   Palme, J.  Common Internet Message Headers.  RFC 2076, February
       1997.

[I5]   Neufeld, G., Baer, J. The Use of URLs as Meta-Syntax for Core
       Mail List Commands and their Transport through Message Header
       Fields.  RFC 2369, July 1998.

[I6]   Crocker, D.  Standard for the format of ARPA Internet text
       messages.  RFC 822, August 1982.

[I7]   Moore, K., Vaudreuil, G.   An Extensible Message Format for
       Delivery Status Notifications.  RFC 1984, January 1996.

[I8]   Fajman, R.  An Extensible Message Format for Message Disposition
       Notifications.  RFC 2298, March 1998.

[I9]   Houttuin, J. BoMBS series: Behavior of Mail Based Servers / Part
       2: A-BoMBS / Answering Servers.  Expired Internet-Draft "draft-
       rare-msg-a-bombs-01.txt", December 1994. (reference included only



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       for attribution)

[I10]  X.400.  (perhaps someone can supply the correct reference for the
       first version of the X.400 document to define autosubmitted?)

[I11]  Kille, S.  MIXER (Mime Internet X.400 Enhanced Relay): Mapping
       between X.400 and RFC 822/MIME.  RFC 2156, January 1998.

[I12]  Palme, J.  "The Auto-Submitted and Expires Headers in E-mail".
       Expired Internet-Draft "draft-ietf-mailext-new-fields-15.txt",
       February 1999. (reference included only for attribution)








































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