The mailto URL scheme
RFC 2368

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:

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     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.

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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

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   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:[email protected]>

   A URL for a mail response system that requires the name of the file
   in the subject:

     <mailto:[email protected]?subject=current-issue>

   A mail response system that requires a "send" request in the body:

     <mailto:[email protected]?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:[email protected]?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:[email protected]?In-Reply-
     To=%[email protected]>

   A request to subscribe to a mailing list:

     <mailto:[email protected]?body=subscribe%20bamboo-l>

   A URL for a single user which includes a CC of another user:

     <mailto:[email protected][email protected]&body=hello>

   Another way of expressing the same thing:

     <mailto:[email protected]&[email protected]&body=hello>

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   Note the use of the "&" reserved character, above. The following
   example, by using "?" twice, is incorrect:

     <mailto:[email protected][email protected]?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%[email protected]" one would do:

     <mailto:gorby%[email protected]>

   To indicate the address "[email protected]", and include
   another header, one would do:

     <mailto:unlikely%[email protected]?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:[email protected]&amp;[email protected]&amp;body=hello">
     mailto:[email protected]&amp;[email protected]&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.

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   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.

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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.

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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

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C. Author Contact Information

   Paul E. Hoffman
   Internet Mail Consortium
   127 Segre Place
   Santa Cruz, CA  95060 USA

   EMail: [email protected]

   Larry Masinter
   Xerox Corporation
   3333 Coyote Hill Road
   Palo Alto, CA 94304 USA

   EMail: [email protected]

   Jamie Zawinski
   Netscape Communications Corp.
   501 East Middlefield Road
   Mountain View, CA 94043 USA

   EMail: [email protected]

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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.

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