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Network Working Group P. Hoffman
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Request for Comments: 2368 Internet Mail Consortium
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Updates: 1738, 1808 L. Masinter
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Category: Standards Track Xerox Corporation
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Netscape Communications
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This document specifies an Internet standards track protocol for the
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Internet community, and requests discussion and suggestions for
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improvements. Please refer to the current edition of the "Internet
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Official Protocol Standards" (STD 1) for the standardization state
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and status of this protocol. Distribution of this memo is unlimited.
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Copyright (C) The Internet Society (1998). All Rights Reserved.
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This document defines the format of Uniform Resource Locators (URL)
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for designating electronic mail addresses. It is one of a suite of
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documents which replace RFC 1738, 'Uniform Resource Locators', and
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RFC 1808, 'Relative Uniform Resource Locators'. The syntax of
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'mailto' URLs from RFC 1738 is extended to allow creation of more RFC
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822 messages by allowing the URL to express additional header and
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The mailto URL scheme is used to designate the Internet mailing
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address of an individual or service. In its simplest form, a mailto
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URL contains an Internet mail address.
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For greater functionality, because interaction with some resources
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may require message headers or message bodies to be specified as well
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as the mail address, the mailto URL scheme is extended to allow
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setting mail header fields and the message body.
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2. Syntax of a mailto URL
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Following the syntax conventions of RFC 1738 [RFC1738], a "mailto"
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mailtoURL = "mailto:" [ to ] [ headers ]
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headers = "?" header *( "&" header )
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header = hname "=" hvalue
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"#mailbox" is as specified in RFC 822 [RFC822]. This means that it
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consists of zero or more comma-separated mail addresses, possibly
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including "phrase" and "comment" components. Note that all URL
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reserved characters in "to" must be encoded: in particular,
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parentheses, commas, and the percent sign ("%"), which commonly occur
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in the "mailbox" syntax.
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"hname" and "hvalue" are encodings of an RFC 822 header name and
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value, respectively. As with "to", all URL reserved characters must
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The special hname "body" indicates that the associated hvalue is the
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body of the message. The "body" hname should contain the content for
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the first text/plain body part of the message. The mailto URL is
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primarily intended for generation of short text messages that are
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actually the content of automatic processing (such as "subscribe"
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messages for mailing lists), not general MIME bodies.
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Within mailto URLs, the characters "?", "=", "&" are reserved.
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Because the "&" (ampersand) character is reserved in HTML, any mailto
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URL which contains an ampersand must be spelled differently in HTML
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than in other contexts. A mailto URL which appears in an HTML
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document must use "&" instead of "&".
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Also note that it is legal to specify both "to" and an "hname" whose
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value is "to". That is,
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mailto:addr1%2C%20addr2
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mailto:?to=addr1%2C%20addr2
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mailto:addr1?to=addr2
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8-bit characters in mailto URLs are forbidden. MIME encoded words (as
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defined in [RFC2047]) are permitted in header values, but not for any
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part of a "body" hname.
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3. Semantics and operations
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A mailto URL designates an "internet resource", which is the mailbox
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specified in the address. When additional headers are supplied, the
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resource designated is the same address, but with an additional
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profile for accessing the resource. While there are Internet
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resources that can only be accessed via electronic mail, the mailto
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URL is not intended as a way of retrieving such objects
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In current practice, resolving URLs such as those in the "http"
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scheme causes an immediate interaction between client software and a
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host running an interactive server. The "mailto" URL has unusual
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semantics because resolving such a URL does not cause an immediate
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interaction. Instead, the client creates a message to the designated
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address with the various header fields set as default. The user can
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edit the message, send this message unedited, or choose not to send
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the message. The operation of how any URL scheme is resolved is not
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mandated by the URL specifications.
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The user agent interpreting a mailto URL SHOULD choose not to create
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a message if any of the headers are considered dangerous; it may also
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choose to create a message with only a subset of the headers given in
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the URL. Only the Subject, Keywords, and Body headers are believed
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to be both safe and useful.
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The creator of a mailto URL cannot expect the resolver of a URL to
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understand more than the "subject" and "body" headers. Clients that
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resolve mailto URLs into mail messages should be able to correctly
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create RFC 822-compliant mail messages using the "subject" and "body"
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RFC 1738 requires that many characters in URLs be encoded. This
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affects the mailto scheme for some common characters that might
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appear in addresses, headers or message contents. One such character
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is space (" ", ASCII hex 20). Note the examples above that use "%20"
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for space in the message body. Also note that line breaks in the
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body of a message MUST be encoded with "%0D%0A".
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People creating mailto URLs must be careful to encode any reserved
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characters that are used in the URLs so that properly-written URL
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interpreters can read them. Also, client software that reads URLs
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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
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understand. These strings should be decoded before showing the user
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The mailto URL scheme is limited in that it does not provide for
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substitution of variables. Thus, a message body that must include a
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user's email address can not be encoded using the mailto URL. This
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limitation also prevents mailto URLs that are signed with public keys
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and other such variable information.
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URLs for an ordinary individual mailing address:
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<mailto:chris@example.com>
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A URL for a mail response system that requires the name of the file
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<mailto:infobot@example.com?subject=current-issue>
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A mail response system that requires a "send" request in the body:
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<mailto:infobot@example.com?body=send%20current-issue>
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A similar URL could have two lines with different "send" requests (in
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this case, "send current-issue" and, on the next line, "send index".)
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<mailto:infobot@example.com?body=send%20current-
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issue%0D%0Asend%20index>
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An interesting use of your mailto URL is when browsing archives of
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messages. Each browsed message might contain a mailto URL like:
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<mailto:foobar@example.com?In-Reply-
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To=%3c3469A91.D10AF4C@example.com>
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A request to subscribe to a mailing list:
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<mailto:majordomo@example.com?body=subscribe%20bamboo-l>
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A URL for a single user which includes a CC of another user:
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<mailto:joe@example.com?cc=bob@example.com&body=hello>
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Another way of expressing the same thing:
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<mailto:?to=joe@example.com&cc=bob@example.com&body=hello>
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Note the use of the "&" reserved character, above. The following
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example, by using "?" twice, is incorrect:
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<mailto:joe@example.com?cc=bob@example.com?body=hello> ; WRONG!
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According to RFC 822, the characters "?", "&", and even "%" may occur
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in addr-specs. The fact that they are reserved characters in this URL
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scheme is not a problem: those characters may appear in mailto URLs,
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they just may not appear in unencoded form. The standard URL encoding
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mechanisms ("%" followed by a two-digit hex number) must be used in
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To indicate the address "gorby%kremvax@example.com" one would do:
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<mailto:gorby%25kremvax@example.com>
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To indicate the address "unlikely?address@example.com", and include
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another header, one would do:
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<mailto:unlikely%3Faddress@example.com?blat=foop>
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As described above, the "&" (ampersand) character is reserved in HTML
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and must be replacded with "&". Thus, a complex URL that has
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internal ampersands might look like:
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<a href="mailto:?to=joe@xyz.com&cc=bob@xyz.com&body=hello">
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mailto:?to=joe@xyz.com&cc=bob@xyz.com&body=hello</a> to
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send a greeting message to <i>Joe and Bob</i>.
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7. Security Considerations
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The mailto scheme can be used to send a message from one user to
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another, and thus can introduce many security concerns. Mail messages
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can be logged at the originating site, the recipient site, and
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intermediary sites along the delivery path. If the messages are not
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encoded, they can also be read at any of those sites.
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A mailto URL gives a template for a message that can be sent by mail
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client software. The contents of that template may be opaque or
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difficult to read by the user at the time of specifying the URL.
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Thus, a mail client should never send a message based on a mailto URL
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without first showing the user the full message that will be sent
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(including all headers that were specified by the mailto URL), fully
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decoded, and asking the user for approval to send the message as
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electronic mail. The mail client should also make it clear that the
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user is about to send an electronic mail message, since the user may
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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
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to the user of what is will be sent; it should disclose not only the
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message destination, but also any headers. Unrecognized headers, or
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headers with values inconsistent with those the mail client would
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normally send should be especially suspect. MIME headers (MIME-
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Version, Content-*) are most likely inappropriate, as are those
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relating to routing (From, Bcc, Apparently-To, etc.)
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Note that some headers are inherently unsafe to include in a message
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generated from a URL. For example, headers such as "From:", "Bcc:",
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and so on, should never be interpreted from a URL. In general, the
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fewer headers interpreted from the URL, the less likely it is that a
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sending agent will create an unsafe message.
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Examples of problems with sending unapproved mail include:
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* mail that breaks laws upon delivery, such as making illegal
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* mail that identifies the sender as someone interested in breaking
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* mail that identifies the sender to an unwanted third party;
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* mail that causes a financial charge to be incurred on the sender;
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* mail that causes an action on the recipient machine that causes
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damage that might be attributed to the sender.
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Programs that interpret mailto URLs should ensure that the SMTP
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"From" address is set and correct.
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8. IANA Considerations
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This document changes the definition of the mailto: URI scheme; any
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registry of URI schemes should refer to this document rather than its
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predecessor, RFC 1738.
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[RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text
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Messages", STD 11, RFC 822, August 1982.
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[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, Editors,
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"Uniform Resource Locators (URL)", RFC 1738, December 1994.
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[RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC
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[RFC2047] Moore, K., "MIME Part Three: Message Header Extensions for
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Non-ASCII Text", RFC 2047, November 1996.
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A. Change from RFC 1738
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RFC 1738 defined only a simple 'mailto' with no headers, just an
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addr-spec (not a full mailbox.) However, required usage and
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implementation has led to the development of an extended syntax that
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included more header fields.
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This document was derived from RFC 1738 and RFC 1808 [RFC1808]; the
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acknowledgments from those specifications still applies.
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The following people contributed to this memo or had and discussed
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similar ideas for mailto.
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C. Author Contact Information
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Internet Mail Consortium
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Santa Cruz, CA 95060 USA
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EMail: phoffman@imc.org
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3333 Coyote Hill Road
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Palo Alto, CA 94304 USA
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EMail: masinter@parc.xerox.com
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Netscape Communications Corp.
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501 East Middlefield Road
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Mountain View, CA 94043 USA
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EMail: jwz@netscape.com
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D. Full Copyright Statement
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Copyright (C) The Internet Society (1998). All Rights Reserved.
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This document and translations of it may be copied and furnished to
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others, and derivative works that comment on or otherwise explain it
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or assist in its implementation may be prepared, copied, published
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and distributed, in whole or in part, without restriction of any
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kind, provided that the above copyright notice and this paragraph are
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included on all such copies and derivative works. However, this
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document itself may not be modified in any way, such as by removing
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the copyright notice or references to the Internet Society or other
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Internet organizations, except as needed for the purpose of
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developing Internet standards in which case the procedures for
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copyrights defined in the Internet Standards process must be
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followed, or as required to translate it into languages other than
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The limited permissions granted above are perpetual and will not be
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revoked by the Internet Society or its successors or assigns.
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This document and the information contained herein is provided on an
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"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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pigeonhole/doc/rfc/mailto.rfc2368.txt
