The mailto URL scheme
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RFC Internet-Draft (individual)
(latest revision 1998-06-09)
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RFC 2368 (Proposed Standard)
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Internet-Draft Paul Hoffman
draft-hoffman-mailto-url-05 Internet Mail Consortium
June 8, 1997 Larry Masinter
Expires in six months Xerox Corporation
The mailto URL scheme
Status of This Memo
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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.
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
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,
is equivalent to
is equivalent to
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.
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
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.
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
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 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
URLs for an ordinary individual mailing address:
A URL for a mail response system that requires the name of the file in the
A mail response system that requires a "send" request in the body:
A similar URL could have two lines with different "send" requests (in this
case, "send current-issue" and, on the next line, "send index".)
An interesting use of your mailto URL is when browsing archives of
messages. Each browsed message might contain a mailto URL like:
A request to subscribe to a mailing list:
A URL for a single user which includes a CC of another user:
Another way of expressing the same thing:
Note the use of the "&" reserved character, above. The following example,
by using "?" twice, is incorrect:
<mailto:firstname.lastname@example.orgemail@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 "firstname.lastname@example.org" one would do:
To indicate the address "email@example.com", and include another
header, one would do:
As described above, the "&" (ampersand) character is reserved in HTML and
must be replacded with "&". Thus, a complex URL that has internal
ampersands might look like:
to send a greeting message to <i>Joe and Bob</i>.
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
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,
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.
This document was derived from RFC 1738 and RFC 1808 [RFC1808]; the
acknowledgments from those specifications still applies.
The following people contributed to this draft or had and discussed similar
ideas for mailto.
[RFC822] Crocker, D., "Standard for the Format of ARPA Internet Text
Messages", RFC 822, University of Delaware, August 1982.
[RFC1738] Berners-Lee, T., Masinter, L., and M. McCahill, Editors, "Uniform
Resource Locators (URL)", RFC 1738, CERN, Xerox Corporation, University of
Minnesota, December 1994.
[RFC1808] Fielding, R., "Relative Uniform Resource Locators", RFC 1808, UC
Irvine, June 1995.
[RFC2047] Moore, K., "MIME Part Three: Message Header Extensions for
Non-ASCII Text", RFC 2047, University of Tennessee, November 1996.
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. Author contact information
Paul E. Hoffman
Internet Mail Consortium
127 Segre Place
Santa Cruz, CA 95060 USA
3333 Coyote Hill Road
Palo Alto, CA 94304 USA
Netscape Communications Corp.
501 East Middlefield Road
Mountain View, CA 94043 USA