Internet Engineering Task Force (IETF) M. Duerst
Request for Comments: 6068 Aoyama Gakuin University
Obsoletes: 2368 L. Masinter
Category: Standards Track Adobe Systems Incorporated
ISSN: 2070-1721 J. Zawinski
October 2010 The 'mailto' URI Scheme
This document defines the format of Uniform Resource Identifiers
(URIs) to identify resources that are reached using Internet mail.
It adds better internationalization and compatibility with
Internationalized Resource Identifiers (IRIs; RFC 3987) to the
previous syntax of 'mailto' URIs (RFC 2368).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 32. Syntax of a 'mailto' URI . . . . . . . . . . . . . . . . . . . 33. Semantics and Operations . . . . . . . . . . . . . . . . . . . 74. Unsafe Header Fields . . . . . . . . . . . . . . . . . . . . . 75. Encoding . . . . . . . . . . . . . . . . . . . . . . . . . . . 86. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 96.1. Basic Examples . . . . . . . . . . . . . . . . . . . . . . 96.2. Examples of Complicated Email Addresses . . . . . . . . . 106.3. Examples Using UTF-8-Based Percent-Encoding . . . . . . . 117. Security Considerations . . . . . . . . . . . . . . . . . . . 128. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 148.1. Update of the Registration of the 'mailto' URI Scheme . . 148.2. Registration of the Body Header Field . . . . . . . . . . 159. Main Changes from RFC 2368 . . . . . . . . . . . . . . . . . . 1510. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 1511. References . . . . . . . . . . . . . . . . . . . . . . . . . . 1611.1. Normative References . . . . . . . . . . . . . . . . . . . 1611.2. Informative References . . . . . . . . . . . . . . . . . . 17
The 'mailto' URI scheme is used to identify resources that are
reached using Internet mail. In its simplest form, a 'mailto' URI
contains an Internet mail address. For interactions that require
message headers or message bodies to be specified, the 'mailto' URI
scheme also allows providing mail header fields and the message body.
This specification extends the previous scheme definition to also
allow character data to be percent-encoded based on UTF-8 [STD63],
which offers a better and more consistent way of dealing with non-
ASCII characters for internationalization.
This specification does not address the needs of the ongoing Email
Address Internationalization effort (see [RFC4952]). In particular,
this specification does not include syntax for fallback addresses.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
In this document, URIs are enclosed in '<' and '>' as described in
Appendix C of [STD66]. Extra whitespace and line breaks are added to
present long URIs -- they are not part of the actual URI.
2. Syntax of a 'mailto' URI
The syntax of a 'mailto' URI is described using the ABNF of [STD68],
non-terminal definitions from [RFC5322] (dot-atom-text, quoted-
string), and non-terminal definitions from [STD66] (unreserved, pct-
mailtoURI = "mailto:" [ to ] [ hfields ]
to = addr-spec *("," addr-spec )
hfields = "?" hfield *( "&" hfield )
hfield = hfname "=" hfvalue
hfname = *qchar
hfvalue = *qchar
addr-spec = local-part "@" domain
local-part = dot-atom-text / quoted-string
domain = dot-atom-text / "[" *dtext-no-obs "]"
dtext-no-obs = %d33-90 / ; Printable US-ASCII
%d94-126 ; characters not including
; "[", "]", or "\"
qchar = unreserved / pct-encoded / some-delims
some-delims = "!" / "$" / "'" / "(" / ")" / "*"
/ "+" / "," / ";" / ":" / "@"
<addr-spec> is a mail address as specified in [RFC5322], but
excluding <comment> from [RFC5322]. However, the following changes
1. A number of characters that can appear in <addr-spec> MUST be
percent-encoded. These are the characters that cannot appear in
a URI according to [STD66] as well as "%" (because it is used for
percent-encoding) and all the characters in gen-delims except "@"
and ":" (i.e., "/", "?", "#", "[", and "]"). Of the characters
in sub-delims, at least the following also have to be percent-
encoded: "&", ";", and "=". Care has to be taken both when
encoding as well as when decoding to make sure these operations
are applied only once.
2. <obs-local-part> and <NO-WS-CTL> as defined in [RFC5322] MUST NOT
3. Whitespace and comments within <local-part> and <domain> MUST NOT
be used. They would not have any operational semantics.
4. Percent-encoding can be used in the <domain> part of an
<addr-spec> in order to denote an internationalized domain name.
The considerations for <reg-name> in [STD66] apply. In
particular, non-ASCII characters MUST first be encoded according
to UTF-8 [STD63], and then each octet of the corresponding UTF-8
sequence MUST be percent-encoded to be represented as URI
characters. URI-producing applications MUST NOT use
percent-encoding in domain names unless it is used to represent a
UTF-8 character sequence. When the internationalized domain name
is used to compose a message, the name MUST be transformed to the
Internationalizing Domain Names in Applications (IDNA) encoding
[RFC5891] where appropriate. URI producers SHOULD provide these
domain names in the IDNA encoding, rather than percent-encoded,
if they wish to maximize interoperability with legacy 'mailto'
5. Percent-encoding of non-ASCII octets in the <local-part> of an
<addr-spec> is reserved for the internationalization of the
<local-part>. Non-ASCII characters MUST first be encoded
according to UTF-8 [STD63], and then each octet of the
corresponding UTF-8 sequence MUST be percent-encoded to be
represented as URI characters. Any other percent-encoding of
non-ASCII characters is prohibited. When a <local-part>
containing non-ASCII characters will be used to compose a
message, the <local-part> MUST be transformed to conform to
whatever encoding may be defined in a future specification for
the internationalization of email addresses.
<hfname> and <hfvalue> are encodings of an [RFC5322] header field
name and value, respectively. Percent-encoding is needed for the
same characters as listed above for <addr-spec>. <hfname> is case-
insensitive, but <hfvalue> in general is case-sensitive. Note that
[RFC5322] allows all US-ASCII printable characters except ":" in
optional header field names (Section 3.6.8), which is the reason why
<pct-encoded> is part of the header field name production.
The special <hfname> "body" indicates that the associated <hfvalue>
is the body of the message. The "body" field value is intended to
contain the content for the first text/plain body part of the
message. The "body" pseudo header field is primarily intended for
the generation of short text messages for automatic processing (such
as "subscribe" messages for mailing lists), not for general MIME
bodies. Except for the encoding of characters based on UTF-8 and
percent-encoding, no additional encoding (such as e.g., base64 or
quoted-printable; see [RFC2045]) is used for the "body" field value.
As a consequence, header fields related to message encoding (e.g.,
Content-Transfer-Encoding) in a 'mailto' URI are irrelevant and MUST
be ignored. The "body" pseudo header field name has been registered
with IANA for this special purpose (see Section 8.2).
Within 'mailto' URIs, the characters "?", "=", and "&" are reserved,
serving as delimiters. They have to be escaped (as "%3F", "%3D", and
"%26", respectively) when not serving as delimiters.
Additional restrictions on what characters are allowed might apply
depending on the context where the URI is used. Such restrictions
can be addressed by context-specific escaping mechanisms. For
example, because the "&" (ampersand) character is reserved in HTML
and XML, any 'mailto' URI that contains an ampersand has to be
written with an HTML/XML entity ("&") or numeric character
reference ("&" or "&").
Non-ASCII characters can be encoded in <hfvalue> as follows:
1. MIME encoded words (as defined in [RFC2047]) are permitted in
header field values, but not in an <hfvalue> of a "body"
<hfname>. Sequences of characters that look like MIME encoded
words can appear in an <hfvalue> of a "body" <hfname>, but in
that case have no special meaning. Please note that the '=' and
'?' characters used as delimiters in MIME encoded words have to
be percent-encoded. Also note that the use of MIME encoded words
differs slightly for so-called structured and unstructured header
2. Non-ASCII characters can be encoded according to UTF-8 [STD63],
and then each octet of the corresponding UTF-8 sequence is
percent-encoded to be represented as URI characters. When header
field values encoded in this way are used to compose a message,
the <hfvalue> has to be suitably encoded (transformed into MIME
encoded words [RFC2047]), except for an <hfvalue> of a "body"
<hfname>, which has to be encoded according to [RFC2045]. Please
note that for MIME encoded words and for bodies in composed email
messages, encodings other than UTF-8 MAY be used as long as the
characters are properly transcoded.
Also note that it is syntactically valid to specify both <to> and an
<hfname> whose value is "to". That is,
is equivalent to
is equivalent to
However, the latter form is NOT RECOMMENDED because different user
agents handle this case differently. In particular, some existing
clients ignore "to" <hfvalue>s.
Implementations MUST NOT produce two "To:" header fields in a
message; the "To:" header field may occur at most once in a message
([RFC5322], Section 3.6). Also, creators of 'mailto' URIs MUST NOT
include other message header fields multiple times if these header
fields can only be used once in a message.
To avoid interoperability problems, creators of 'mailto' URIs SHOULD
NOT use the same <hfname> multiple times in the same URI. If the
same <hfname> appears multiple times in a URI, behavior varies widely
for different user agents, and for each <hfname>. Examples include
using only the first or last <hfname>/<hfvalue> pair, creating
multiple header fields, and combining each <hfvalue> by simple
concatenation or in a way appropriate for the corresponding header
Note that this specification, like any URI scheme specification, does
not define syntax or meaning of a fragment identifier (see [STD66]),
because these depend on the type of a retrieved representation. In
the currently known usage scenarios, a 'mailto' URI cannot be used to
retrieve such representations. Therefore, fragment identifiers are
meaningless, SHOULD NOT be used on 'mailto' URIs, and SHOULD be
ignored upon resolution. The character "#" in <hfvalue>s MUST be
escaped as %23.
3. Semantics and Operations
A 'mailto' URI designates an "Internet resource", which is the
mailbox specified in the address. When additional header fields 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' URI is not intended as a way of retrieving such objects
The operation of how any URI scheme is resolved is not mandated by
the URI specifications. In current practice, resolving URIs such as
those in the 'http' URI scheme causes an immediate interaction
between client software and a host running an interactive server.
The 'mailto' URI has unusual semantics because resolving such a URI
does not cause an immediate interaction with a server. 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
the message unedited, or choose not to send the message.
The <hfname>/<hfvalue> pairs in a 'mailto' URI, although
syntactically equivalent to header fields in a mail message, do not
directly correspond to the header fields in a mail message. In
particular, the To, Cc, and Bcc <hfvalue>s don't necessarily result
in a header field containing the specified value. Mail client
software MAY eliminate duplicate addresses. Creators of 'mailto'
URIs SHOULD avoid using the same address twice in a 'mailto' URI.
Originator fields like From and Date, fields related to routing
(Apparently-To, Resent-*, etc.), trace fields, and MIME header fields
(MIME-Version, Content-*), when present in the URI, MUST be ignored.
The mail client MUST create new fields when necessary, as it would
for any new message. Unrecognized header fields and header fields
with values inconsistent with those the mail client would normally
send SHOULD be treated as especially suspect. For example, there may
be header fields that are totally safe but not known to the MUA, so
the MUA MAY choose to show them to the user.
4. Unsafe Header Fields
The user agent interpreting a 'mailto' URI SHOULD NOT create a
message if any of the header fields are considered dangerous; it MAY
also choose to create a message with only a subset of the header
fields given in the URI. Only a limited set of header fields such as
Subject and Keywords, as well as Body, are believed to be both safe
and useful in the general case. In cases where the source of a URI
is well known, and/or specific header fields are limited to specific
well-known values, other header fields MAY be considered safe, too.
The creator of a 'mailto' URI cannot expect the resolver of a URI to
understand more than the "subject" header field and "body". Clients
that resolve 'mailto' URIs into mail messages MUST be able to
correctly create [RFC5322]-compliant mail messages using the
"subject" header field and "body".
[STD66] requires that many characters in URIs be encoded. This
affects the 'mailto' URI scheme for some common characters that might
appear in addresses, header fields, or message contents. One such
character is space (" ", ASCII hex 20). Note the examples below 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".
Implementations MAY add a final line break to the body of a message
even if there is no trailing "%0D%0A" in the body <hfield> of the
'mailto' URI. Line breaks in other <hfield>s SHOULD NOT be used.
When creating 'mailto' URIs, any reserved characters that are used in
the URIs MUST be encoded so that properly written URI interpreters
can read them. Also, client software that reads URIs MUST decode
strings before creating the mail message so that the mail message
appears in a form that the recipient software will understand. These
strings SHOULD be decoded before showing the message to the sending
Software creating 'mailto' URIs likewise has to be careful to encode
any reserved characters that are used. HTML forms are one kind of
software that creates 'mailto' URIs. Current implementations encode
a space as '+', but this creates problems because such a '+' standing
for a space cannot be distinguished from a real '+' in a 'mailto'
URI. When producing 'mailto' URIs, all spaces SHOULD be encoded as
%20, and '+' characters MAY be encoded as %2B. Please note that '+'
characters are frequently used as part of an email address to
indicate a subaddress, as for example in <firstname.lastname@example.org>.
The 'mailto' URI scheme is limited in that it does not provide for
substitution of variables. Thus, it is impossible to create a
'mailto' URI that includes a user's email address in the message
body. This limitation also prevents 'mailto' URIs that are signed
with public keys and other such variable information.
6.1. Basic Examples
A URI for an ordinary individual mailing address:
A URI for a mail response system that requires the name of the file
to be sent back in the subject:
A mail response system that requires a "send" request in the body:
A similar URI, with two lines with different "send" requests (in this
case, "send current-issue" and, on the next line, "send index"):
An interesting use of 'mailto' URIs occurs when browsing archives of
messages. A link can be provided that allows replying to a message
and conserving threading information. This is done by adding an
In-Reply-To header field containing the Message-ID of the message
where the link is added, for example:
A request to subscribe to a mailing list:
A URI that is for a single user and that includes a CC of another
Note the use of the "&" reserved character above. The following
example, using "?" twice, is incorrect:
<mailto:email@example.comfirstname.lastname@example.org?body=hello> ; WRONG!
According to [RFC5322], the characters "?", "&", and even "%" may
occur in addr-specs. The fact that they are reserved characters is
not a problem: those characters may appear in 'mailto' URIs -- they
just may not appear in unencoded form. The standard URI encoding
mechanisms ("%" followed by a two-digit hex number) MUST be used in
To indicate the address "email@example.com" one would use:
To indicate the address "firstname.lastname@example.org", and include
another header field, one would use:
As described above, the "&" (ampersand) character is reserved in HTML
and has to be replaced, e.g., with "&". Thus, a URI with an
internal ampersand might look like:
>mailto:email@example.comfirstname.lastname@example.org&body=hello</a> to send a
greeting message to Joe and Bob.
When an email address itself includes an "&" (ampersand) character,
that character has to be percent-encoded. For example, the 'mailto'
URI to send mail to "Mikeemail@example.com" is
6.2. Examples of Complicated Email Addresses
Following are a few examples of how to treat email addresses that
contain complicated escaping syntax.
Email address: "not@me"@example.org; corresponding 'mailto' URI:
Email address: "oh\\no"@example.org; corresponding 'mailto' URI:
Email address: "\\\"it's\ ugly\\\""@example.org; corresponding
6.3. Examples Using UTF-8-Based Percent-Encoding
Sending a mail with the subject "coffee" in French, i.e., "cafe"
where the final e is an e-acute, using UTF-8 and percent-encoding:
The same subject, this time using an encoded-word (escaping the "="
and "?" characters used in the encoded-word syntax, because they are
The same subject, this time encoded as iso-8859-1:
Going back to straight UTF-8 and adding a body with the same value:
This 'mailto' URI may result in a message looking like this:
The software sending the email is not restricted to UTF-8, but can
use other encodings. The following shows the same email using
iso-8859-1 two times:
Different content transfer encodings (i.e., "8bit" or "base64"
instead of "quoted-printable") and different encodings in encoded
words (i.e., "B" instead of "Q") can also be used.
For more examples of encoding the word coffee in different languages,
The following example uses the Japanese word "natto" (Unicode
characters U+7D0D U+8C46) as a domain name label, sending a mail to a
user at "natto".example.org:
When constructing the email, the domain name label is converted to
punycode. The resulting message may look as follows:
7. Security Considerations
The 'mailto' URI 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 encrypted, they can also be read at any of those sites.
A 'mailto' URI 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 URI, as
well as being hidden in the user interface (for example, a link on an
HTML Web page might display something other than the content of the
corresponding 'mailto' URI that would be used when clicked). Thus, a
mail client SHOULD NOT send a message based on a 'mailto' URI without
first disclosing and showing to the user the full message that will
be sent (including all header fields that were specified by the
'mailto' URI), 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' URI. Users are strongly encouraged to ensure that the
'mailto' URI presented to them matches the address included in the
"To:" line of the email message.
Some header fields are inherently unsafe to include in a message
generated from a URI. For details, please see Section 3. In
general, the fewer header fields interpreted from the URI, 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
mail that identifies the sender as someone interested in breaking
mail that identifies the sender to an unwanted third party;
mail that causes a financial charge to be incurred by 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' URIs SHOULD ensure that the SMTP
envelope return path address, which is given as an argument to the
SMTP MAIL FROM command, is set and correct, and that the resulting
email is a complete, workable message.
'mailto' URIs on public Web pages expose mail addresses for
harvesting. This applies to all mail addresses that are part of the
'mailto' URI, including the addresses in a "bcc" <hfvalue>. Those
addresses will not be sent to the recipients in the 'to' field and in
the "to" and "cc" <hfvalue>s, but will still be publicly visible in
the URI. Addresses in a "bcc" <hfvalue> may also leak to other
addresses in the same <hfvalue> or become known otherwise, depending
on the mail user agent used.
Programs manipulating 'mailto' URIs have to take great care to not
inadvertently double-escape or double-unescape 'mailto' URIs, and to
make sure that escaping and unescaping conventions relating to URIs
and relating to mail addresses are applied in the right order.
Implementations parsing 'mailto' URIs must take care to sanity check
'mailto' URIs in order to avoid buffer overflows and problems
resulting from them (e.g., execution of code specified by the
The security considerations of [STD66], [RFC5890], [RFC5891], and
[RFC3987] also apply. Implementers and users are advised to check
8. IANA Considerations
8.1. Update of the Registration of the 'mailto' URI Scheme
This document changes the definition of the 'mailto' URI scheme; the
registry of URI schemes has been updated to refer to this document
rather than its predecessor, [RFC2368]. The registration template is
URI scheme name:
URI scheme syntax:
See the syntax section of RFC 6068.
URI scheme semantics:
See the semantics section of RFC 6068.
See the syntax and encoding sections of RFC 6068.
Applications/protocols that use this URI scheme name:
The 'mailto' URI scheme is widely used since the start of the Web.
Interoperability for 'mailto' URIs with UTF-8-based percent-
encoding might be somewhat lower than interoperability for
'mailto' URIs with US-ASCII only.
See the security considerations section of RFC 6068.
8.2. Registration of the Body Header Field
IANA has registered the Body header field in the Message Header
Fields Registry ([RFC3864]) as follows:
Header field name:
None. This registration is made to assure that this header field
name is not used at all, in order to not create any problems for
9. Main Changes from RFC 2368
The main changes from RFC 2368 are as follows:
o Changed syntax from RFC 2822 <mailbox> to [RFC5322] <addr-spec>.
o Allowed UTF-8-based percent-encoding for domain names and in
o Nailed down percent-encoding in <local-part> to be based on UTF-8,
reserved for if and when there is a specification for the
internationalization of email addresses.
o Removed prohibition against "Bcc:" header fields, but added a
warning about their visibility and harvesting for spam.
o Added clarifications for escaping.
This document was derived from [RFC2368]; the acknowledgments from
that specification still apply. In addition, we thank Paul Hoffman
for his work on [RFC2368].
Valuable input on this document was received from (in no particular
order): Alexey Melnikov, Paul Hoffman, Charles Lindsey, Tim Kindberg,
Frank Ellermann, Etan Wexler, Michael Haardt, Michael Anthony
Puls II, Eliot Lear, Dave Crocker, Dan Harkins, Nevil Brownlee, John
Klensin, Alfred Hoenes, Ned Freed, Sean Turner, Peter Saint-Andre,
Adrian Farrel, Avshalom Houri, Robert Sparks, and many others.
11.1. Normative References
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, November 1996.
[RFC2047] Moore, K., "MIME Part Three: Message Header Extensions for
Non-ASCII Text", RFC 2047, November 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3864] Klyne, G., Nottingham, M., and J. Mogul, "Registration
Procedures for Message Header Fields", BCP 90, RFC 3864,
[RFC3987] Duerst, M. and M. Suignard, "Internationalized Resource
Identifiers (IRIs)", RFC 3987, January 2005.
[RFC5322] Resnik, P., "Internet Message Format", RFC 5322,
[RFC5890] Klensin, J., "Internationalized Domain Names for
Applications (IDNA): Definitions and Document Framework",
RFC 5890, August 2010.
[RFC5891] Klensin, J., "Internationalized Domain Names in
Applications (IDNA): Protocol", RFC 5891, August 2010.
[STD63] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[STD66] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[STD68] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
11.2. Informative References
[RFC2324] Masinter, L., "Hyper Text Coffee Pot Control Protocol
(HTCPCP/1.0)", RFC 2324, April 1998.
[RFC2368] Hoffman, P., Masinter, L., and J. Zawinski, "The mailto
URL scheme", RFC 2368, July 1998.
[RFC4952] Klensin, J. and Y. Ko, "Overview and Framework for
Internationalized Email", RFC 4952, July 2007.
Martin Duerst (Note: Please write "Duerst" with u-umlaut wherever
possible, for example as "Dürst" in XML and HTML.)
Aoyama Gakuin University
Sagamihara, Kanagawa 229-8558
Phone: +81 42 759 6329
Fax: +81 42 759 6495
Adobe Systems Incorporated
345 Park Ave
San Jose, CA 95110
375 Eleventh Street
San Francisco, CA 94103