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rfc:rfc2368

Network Working Group P. Hoffman Request for Comments: 2368 Internet Mail Consortium Updates: 1738, 1808 L. Masinter Category: Standards Track Xerox Corporation

                                                         J. Zawinski
                                             Netscape Communications
                                                           July 1998
                       The mailto URL scheme

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 (1998).  All Rights Reserved.

Abstract

 This document defines the format of Uniform Resource Locators (URL)
 for designating electronic mail addresses. It is one of a suite of
 documents which replace RFC 1738, 'Uniform Resource Locators', and
 RFC 1808, 'Relative Uniform Resource Locators'. The syntax of
 'mailto' URLs from RFC 1738 is extended to allow creation of more RFC
 822 messages by allowing the URL to express additional header and
 body fields.

1. Introduction

 The mailto URL scheme is used to designate the Internet mailing
 address of an individual or service. In its simplest form, a mailto
 URL contains an Internet mail address.
 For greater functionality, because interaction with some resources
 may require message headers or message bodies to be specified as well
 as the mail address, the mailto URL scheme is extended to allow
 setting mail header fields and the message body.

2. Syntax of a mailto URL

 Following the syntax conventions of RFC 1738 [RFC1738], a "mailto"
 URL has the form:

Hoffman, et. al. Standards Track [Page 1] RFC 2368 The mailto URL scheme July 1998

   mailtoURL  =  "mailto:" [ to ] [ headers ]
   to         =  #mailbox
   headers    =  "?" header *( "&" header )
   header     =  hname "=" hvalue
   hname      =  *urlc
   hvalue     =  *urlc
 "#mailbox" is as specified in RFC 822 [RFC822]. This means that it
 consists of zero or more comma-separated mail addresses, possibly
 including "phrase" and "comment" components. Note that all URL
 reserved characters in "to" must be encoded: in particular,
 parentheses, commas, and the percent sign ("%"), which commonly occur
 in the "mailbox" syntax.
 "hname" and "hvalue" are encodings of an RFC 822 header name and
 value, respectively. As with "to", all URL reserved characters must
 be encoded.
 The special hname "body" indicates that the associated hvalue is the
 body of the message. The "body" hname should contain the content for
 the first text/plain body part of the message. The mailto URL is
 primarily intended for generation of short text messages that are
 actually the content of automatic processing (such as "subscribe"
 messages for mailing lists), not general MIME bodies.
 Within mailto URLs, the characters "?", "=", "&" are reserved.
 Because the "&" (ampersand) character is reserved in HTML, any mailto
 URL which contains an ampersand must be spelled differently in HTML
 than in other contexts.  A mailto URL which appears in an HTML
 document must use "&" instead of "&".
 Also note that it is legal to specify both "to" and an "hname" whose
 value is "to". That is,
   mailto:addr1%2C%20addr2
   is equivalent to
   mailto:?to=addr1%2C%20addr2
   is equivalent to
   mailto:addr1?to=addr2
 8-bit characters in mailto URLs are forbidden. MIME encoded words (as
 defined in [RFC2047]) are permitted in header values, but not for any
 part of a "body" hname.

Hoffman, et. al. Standards Track [Page 2] RFC 2368 The mailto URL scheme July 1998

3. Semantics and operations

 A mailto URL designates an "internet resource", which is the mailbox
 specified in the address. When additional headers are supplied, the
 resource designated is the same address, but with an additional
 profile for accessing the resource. While there are Internet
 resources that can only be accessed via electronic mail, the mailto
 URL is not intended as a way of retrieving such objects
 automatically.
 In current practice, resolving URLs such as those in the "http"
 scheme causes an immediate interaction between client software and a
 host running an interactive server. The "mailto" URL has unusual
 semantics because resolving such a URL does not cause an immediate
 interaction. Instead, the client creates a message to the designated
 address with the various header fields set as default. The user can
 edit the message, send this message unedited, or choose not to send
 the message. The operation of how any URL scheme is resolved is not
 mandated by the URL specifications.

4. Unsafe headers

 The user agent interpreting a mailto URL SHOULD choose not to create
 a message if any of the headers are considered dangerous; it may also
 choose to create a message with only a subset of the headers given in
 the URL.  Only the Subject, Keywords, and Body headers are believed
 to be both safe and useful.
 The creator of a mailto URL cannot expect the resolver of a URL to
 understand more than the "subject" and "body" headers. Clients that
 resolve mailto URLs into mail messages should be able to correctly
 create RFC 822-compliant mail messages using the "subject" and "body"
 headers.

5. Encoding

 RFC 1738 requires that many characters in URLs be encoded. This
 affects the mailto scheme for some common characters that might
 appear in addresses, headers or message contents. One such character
 is space (" ", ASCII hex 20). Note the examples above that use "%20"
 for space in the message body.  Also note that line breaks in the
 body of a message MUST be encoded with "%0D%0A".
 People creating mailto URLs must be careful to encode any reserved
 characters that are used in the URLs so that properly-written URL
 interpreters can read them. Also, client software that reads URLs
 must be careful to decode strings before creating the mail message so

Hoffman, et. al. Standards Track [Page 3] RFC 2368 The mailto URL scheme July 1998

 that the mail messages appear in a form that the recipient will
 understand. These strings should be decoded before showing the user
 the message.
 The mailto URL scheme is limited in that it does not provide for
 substitution of variables. Thus, a message body that must include a
 user's email address can not be encoded using the mailto URL. This
 limitation also prevents mailto URLs that are signed with public keys
 and other such variable information.

6. Examples

 URLs for an ordinary individual mailing address:
   <mailto:chris@example.com>
 A URL for a mail response system that requires the name of the file
 in the subject:
   <mailto:infobot@example.com?subject=current-issue>
 A mail response system that requires a "send" request in the body:
   <mailto:infobot@example.com?body=send%20current-issue>
 A similar URL could have two lines with different "send" requests (in
 this case, "send current-issue" and, on the next line, "send index".)
   <mailto:infobot@example.com?body=send%20current-
   issue%0D%0Asend%20index>
 An interesting use of your mailto URL is when browsing archives of
 messages. Each browsed message might contain a mailto URL like:
   <mailto:foobar@example.com?In-Reply-
   To=%3c3469A91.D10AF4C@example.com>
 A request to subscribe to a mailing list:
   <mailto:majordomo@example.com?body=subscribe%20bamboo-l>
 A URL for a single user which includes a CC of another user:
   <mailto:joe@example.com?cc=bob@example.com&body=hello>
 Another way of expressing the same thing:
   <mailto:?to=joe@example.com&cc=bob@example.com&body=hello>

Hoffman, et. al. Standards Track [Page 4] RFC 2368 The mailto URL scheme July 1998

 Note the use of the "&" reserved character, above. The following
 example, by using "?" twice, is incorrect:
   <mailto:joe@example.com?cc=bob@example.com?body=hello>   ; WRONG!
 According to RFC 822, the characters "?", "&", and even "%" may occur
 in addr-specs. The fact that they are reserved characters in this URL
 scheme is not a problem: those characters may appear in mailto URLs,
 they just may not appear in unencoded form. The standard URL encoding
 mechanisms ("%" followed by a two-digit hex number) must be used in
 certain cases.
 To indicate the address "gorby%kremvax@example.com" one would do:
   <mailto:gorby%25kremvax@example.com>
 To indicate the address "unlikely?address@example.com", and include
 another header, one would do:
   <mailto:unlikely%3Faddress@example.com?blat=foop>
 As described above, the "&" (ampersand) character is reserved in HTML
 and must be replacded with "&amp;". Thus, a complex URL that has
 internal ampersands might look like:
   Click
   <a href="mailto:?to=joe@xyz.com&amp;cc=bob@xyz.com&amp;body=hello">
   mailto:?to=joe@xyz.com&amp;cc=bob@xyz.com&amp;body=hello</a> to
   send a greeting message to <i>Joe and Bob</i>.

7. Security Considerations

 The mailto scheme can be used to send a message from one user to
 another, and thus can introduce many security concerns. Mail messages
 can be logged at the originating site, the recipient site, and
 intermediary sites along the delivery path. If the messages are not
 encoded, they can also be read at any of those sites.
 A mailto URL gives a template for a message that can be sent by mail
 client software. The contents of that template may be opaque or
 difficult to read by the user at the time of specifying the URL.
 Thus, a mail client should never send a message based on a mailto URL
 without first showing the user the full message that will be sent
 (including all headers that were specified by the mailto URL), fully
 decoded, and asking the user for approval to send the message as
 electronic mail. The mail client should also make it clear that the
 user is about to send an electronic mail message, since the user may
 not be aware that this is the result of a mailto URL.

Hoffman, et. al. Standards Track [Page 5] RFC 2368 The mailto URL scheme July 1998

 A mail client should never send anything without complete disclosure
 to the user of what is will be sent; it should disclose not only the
 message destination, but also any headers. Unrecognized headers, or
 headers with values inconsistent with those the mail client would
 normally send should be especially suspect. MIME headers (MIME-
 Version, Content-*) are most likely inappropriate, as are those
 relating to routing (From, Bcc, Apparently-To, etc.)
 Note that some headers are inherently unsafe to include in a message
 generated from a URL. For example, headers such as "From:", "Bcc:",
 and so on, should never be interpreted from a URL. In general, the
 fewer headers interpreted from the URL, the less likely it is that a
 sending agent will create an unsafe message.
 Examples of problems with sending unapproved mail include:
  • mail that breaks laws upon delivery, such as making illegal

threats;

  • mail that identifies the sender as someone interested in breaking

laws;

  • mail that identifies the sender to an unwanted third party;
  • mail that causes a financial charge to be incurred on the sender;
  • mail that causes an action on the recipient machine that causes

damage that might be attributed to the sender.

 Programs that interpret mailto URLs should ensure that the SMTP
 "From" address is set and correct.

8. IANA Considerations

 This document changes the definition of the mailto: URI scheme; any
 registry of URI schemes should refer to this document rather than its
 predecessor, RFC 1738.

Hoffman, et. al. Standards Track [Page 6] RFC 2368 The mailto URL scheme July 1998

9. References

 [RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text
          Messages", STD 11, RFC 822, August 1982.
 [RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, Editors,
           "Uniform Resource Locators (URL)", RFC 1738, December 1994.
 [RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC
           1808, June 1995.
 [RFC2047] Moore, K., "MIME Part Three: Message Header Extensions for
           Non-ASCII Text", RFC 2047, November 1996.

Hoffman, et. al. Standards Track [Page 7] RFC 2368 The mailto URL scheme July 1998

A. Change from RFC 1738

 RFC 1738 defined only a simple 'mailto' with no headers, just an
 addr-spec (not a full mailbox.)  However, required usage and
 implementation has led to the development of an extended syntax that
 included more header fields.

B. Acknowledgments

 This document was derived from RFC 1738 and RFC 1808 [RFC1808]; the
 acknowledgments from those specifications still applies.
 The following people contributed to this memo or had and discussed
 similar ideas for mailto.
 Harald Alvestrand
 Bryan Costales
 Steve Dorner
 Al Gilman
 Mark Joseph
 Laurence Lundblade
 Keith Moore
 Jacob Palme
 Michael Patton

Hoffman, et. al. Standards Track [Page 8] RFC 2368 The mailto URL scheme July 1998

C. Author Contact Information

 Paul E. Hoffman
 Internet Mail Consortium
 127 Segre Place
 Santa Cruz, CA  95060 USA
 EMail: phoffman@imc.org
 Larry Masinter
 Xerox Corporation
 3333 Coyote Hill Road
 Palo Alto, CA 94304 USA
 EMail: masinter@parc.xerox.com
 Jamie Zawinski
 Netscape Communications Corp.
 501 East Middlefield Road
 Mountain View, CA 94043 USA
 EMail: jwz@netscape.com

Hoffman, et. al. Standards Track [Page 9] RFC 2368 The mailto URL scheme July 1998

D. Full Copyright Statement

 Copyright (C) The Internet Society (1998).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Hoffman, et. al. Standards Track [Page 10]

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