The 'mailto' URI/IRI Scheme
draft-duerst-eai-mailto-03

Abstract

This document defines the format of Uniform Resource Identifiers (URIs) and Internationalized Resource Identfiers (IRIs) to identify resources that are reached using Internet mail. It adds the possibility to use Email Address Internationalization (EAI) email addresses (RFC6530) to the previous syntax of 'mailto' URIs (RFC 6068).

Status of this Memo

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This Internet-Draft will expire on September 27, 2012.

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Table of Contents

1.  Introduction
2.  Syntax of a 'mailto' URI
    2.1.  Syntax Rules
    2.2.  Additional Details about <addr-spec-enc>
    2.3.  Additional Details about <hfname> and <hfvalue>
3.  Semantics and Operations
4.  Unsafe Header Fields
5.  Encoding
6.  Examples
    6.1.  Conventions Used
    6.2.  Basic Examples
    6.3.  Examples of Complicated Email Addresses
    6.4.  Examples Using UTF-8-Based Percent-Encoding usable with RFC 5322
    6.5.  Examples Using UTF-8-Based Percent-Encoding usable only with EAI
7.  Security Considerations
8.  IANA Considerations
    8.1.  Update of the Registration of the 'mailto' URI/IRI Scheme
    8.2.  Registration of the Body Header Field
9.  Main Changes from RFC 6068
10.  Change record
    10.1.  Changes from -02 to -03
    10.2.  Changes from -01 to -02
    10.3.  Changes from -00 to -01
    10.4.  Changes from RFC 6068 to -00
11.  Acknowledgments
12.  References
    12.1.  Normative References
    12.2.  Informative References
§  Authors' Addresses

1.  Introduction

The 'mailto' URI/IRI scheme [RFC4395bis] (Hansen, T., Hardie, T., and L. Masinter, “Guidelines and Registration Procedures for New URI/IRI Schemes,” December 2011.) is used to identify resources that are reached using Internet mail. In its simplest form, a 'mailto' URI/IRI contains an Internet mail address. For interactions that require message headers or message bodies to be specified, the 'mailto' URI/IRI scheme also allows providing mail header fields and the message body.

This specification extends the previous scheme definition ([RFC6068] (Dürst, M., Masinter, L., and J. Zawinski, “The 'mailto' URI Scheme,” October 2010.)) to also allow non-ASCII characters in the left-hand sides (LHSs) of email addresses. To work seamlessly with Internationalized Resource Identfiers (IRIs, [RFC3987] (Dürst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.)) and Email Address Internationalization (EAI, [RFC6530] (Klensin, J. and Y. Ko (고양우), “Overview and Framework for Internationalized Email,” February 2012.)), these LHSs are percent-encoded based on UTF-8 [STD63] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) when used in URIs.

This document is available in (line-printer ready) plaintext ASCII and PDF. It is also available in HTML from http://www.sw.it.aoyama.ac.jp/2012/pub/draft-duerst-eai-mailto-03.html While all these versions are identical in their technical content, the HTML and PDF versions show non-Unicode characters directly. This often makes it easier to understand examples, and readers are therefore advised to consult these versions in preference or as a supplement to the ASCII version.

Example URIs and IRIs are enclosed in '<' and '>' as described in Appendix C of [STD66] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.). Extra whitespace and line breaks are added to present long URIs -- they are not part of the actual URI.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

2.  Syntax of a 'mailto' URI

2.1.  Syntax Rules

The syntax of a 'mailto' URI is described using the ABNF of [STD68] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.). The syntax of a 'mailto' IRI can be obtained from this definition by allowing <iunreserved> characters wherever <unreserved> characters are allowed. The syntax below also uses non-terminal definitions from [STD66] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) (unreserved, pct-encoded):

   mailtoURI         = "mailto:" [ to ] [ hfields ]
   to                = addr-spec-enc *("," addr-spec-enc )
   hfields           = "?" hfield *( "&" hfield )
   hfield            = hfname "=" hfvalue
   hfname            = *qchar
   hfvalue           = *qchar
   addr-spec-enc     = local-part-enc "@" domain-enc
   local-part-enc    = dot-atom-text-enc / quoted-string-enc
   domain-enc        = dot-atom-text-enc / "[" *dtext-no-obs "]"
   dtext-no-obs      = %d33-90  ; Printable US-ASCII
                     / %d94-126 ; characters not including
                                ; "[", "]", or "\"
   dot-atom-text-enc = <percent-encoded version of
                        dot-atom-text or its EAI equivalent>
   quoted-string-enc = <percent-encoded version of
                        dot-atom-text or its EAI equivalent>
   qchar             = unreserved / pct-encoded / some-delims
   some-delims       = "!" / "$" / "'" / "(" / ")" / "*"
                     / "+" / "," / ";" / ":" / "@" / "/" / "?"

In addition to the above syntax rules, the details given in the next two subsections are relevant.

2.2.  Additional Details about <addr-spec-enc>

<addr-spec-enc> is a mail address as specified by <addr-spec> in [RFC5322] (Resnik, P., “Internet Message Format,” October 2008.) or <uAddr-Spec> in [RFC6532] (Yang, A., Steele, S., and N. Freed, “Internationalized Email Headers,” February 2012.), but excluding <comment>, with 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] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) 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] (Resnik, P., “Internet Message Format,” October 2008.) MUST NOT be used.
3. Whitespace and comments within <local-part-enc> and <domain-enc> MUST NOT be used. They would not have any operational semantics.
4. Percent-encoding can be used in the <domain-enc> part of an <addr-spec-enc>, in order to denote an internationalized domain name. The considerations for <reg-name> in [STD66] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) apply. In particular, non-ASCII characters MUST first be encoded according to UTF-8 [STD63] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.), 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] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” August 2010.) 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' URI interpreters.
5. Percent-encoding of non-ASCII octets in the <local-part-enc> of an <addr-spec-enc> is used for the internationalization of the <local-part-enc> according to Email Address Internationalization (EAI; [RFC6532] (Yang, A., Steele, S., and N. Freed, “Internationalized Email Headers,” February 2012.)). Non-ASCII characters MUST first be encoded according to UTF-8 [STD63] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.), 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-enc> containing non-ASCII characters will be used to compose a message, the <local-part-enc> MUST be transformed back to UTF-8 in order to conform to EAI.

<dot-atom-text-enc> is the percent-encoded version of <dot-atom-text> in [RFC5322] (Resnik, P., “Internet Message Format,” October 2008.) or <uDot-Atom-text> in [RFC6532] (Yang, A., Steele, S., and N. Freed, “Internationalized Email Headers,” February 2012.). <quoted-string-enc> is the percent-encoded version of <quoted-string> in [RFC5322] (Resnik, P., “Internet Message Format,” October 2008.) or <uQuoted-String> in [RFC6532] (Yang, A., Steele, S., and N. Freed, “Internationalized Email Headers,” February 2012.).

2.3.  Additional Details about <hfname> and <hfvalue>

<hfname> and <hfvalue> are encodings of an [RFC5322] (Resnik, P., “Internet Message Format,” October 2008.) header field name and value, respectively. Percent-encoding is needed for the same characters as listed above for <addr-spec-enc>. <hfname> is case-insensitive, but <hfvalue> in general is case-sensitive. Note that [RFC5322] (Resnik, P., “Internet Message Format,” October 2008.) 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] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.)) 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 (Registration of the Body Header Field)).

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 ("&amp;") or numeric character reference ("&#x26;" or "&#38;").

Non-ASCII characters can be encoded in <hfvalue> as follows:

1. MIME encoded words (as defined in [RFC2047] (Moore, K., “MIME Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.)) 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 fields.
2. Non-ASCII characters MUST be encoded according to UTF-8 [STD63] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) , 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 conforming to [RFC5322] (Resnik, P., “Internet Message Format,” October 2008.), the <hfvalue> has to be suitably encoded (transformed into MIME encoded words [RFC2047] (Moore, K., “MIME Part Three: Message Header Extensions for Non-ASCII Text,” November 1996.)), except for an <hfvalue> of a "body" <hfname>, which has to be encoded according to [RFC2045] (Freed, N. and N. Borenstein, “Multipurpose Internet Mail Extensions (MIME) Part One: Format of Internet Message Bodies,” November 1996.). 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. When header field values encoded in this way are used to compose a message conforming to [RFC6532] (Yang, A., Steele, S., and N. Freed, “Internationalized Email Headers,” February 2012.), percent-encoding (including reserved characters) has to be decoded. The header field values can then be used directly because EAI allows UTF-8 in header field values.

Also note that it is syntactically valid to specify both <to> and an <hfname> whose value is "to". That is,

<mailto:addr1@an.example,addr2@an.example>

is equivalent to

<mailto:?to=addr1@an.example,addr2@an.example>

is equivalent to

<mailto:addr1@an.example?to=addr2@an.example>

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] (Resnik, P., “Internet Message Format,” October 2008.), 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 field.

Note that this specification, like any URI scheme specification, does not define syntax or meaning of a fragment identifier (see [STD66] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.)), 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/IRI 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 automatically.

The operation of how any URI/IRI scheme is resolved is not mandated by the URI specifications. In current practice, resolving URIs/IRIs such as those in the 'http' URI/IRI scheme causes an immediate interaction between client software and a host running an interactive server. The 'mailto' URI/IRI has unusual semantics because resolving such a URI/IRI 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/IRI, 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/IRI.

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/IRI, 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/IRI 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/IRI. 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/IRI 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/IRI cannot expect the resolver of a URI/IRI to understand more than the "subject" header field and "body". Clients that resolve 'mailto' URIs/IRIs into mail messages MUST be able to correctly create [RFC5322] (Resnik, P., “Internet Message Format,” October 2008.)-compliant mail messages using the "subject" header field and "body".

5.  Encoding

[STD66] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) requires that many characters in URIs/IRIs be encoded. This affects the 'mailto' URI/IRI 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/IRI. Line breaks in other <hfield>s SHOULD NOT be used.

When creating 'mailto' URIs/IRIs, any reserved characters that are used in the URIs/IRIs MUST be encoded so that properly written URI/IRI interpreters can read them. Also, client software that reads URIs/IRIs 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 user.

Software creating 'mailto' URIs/IRIs likewise has to be careful to encode any reserved characters that are used. HTML forms are one kind of software that creates 'mailto' URIs/IRIs. 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/IRI. When producing 'mailto' URIs/IRIs, 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 <bill+ietf@example.org>.

The 'mailto' URI/IRI scheme is limited in that it does not provide for substitution of variables. Thus, it is impossible to create a 'mailto' URI/IRI that includes a user's email address in the message body. This limitation also prevents 'mailto' URIs/IRIs that are signed with public keys and other such variable information.

6.  Examples

6.1.  Conventions Used

To represent characters outside US-ASCII in a document format that is limited to US-ASCII, this document uses 'XML Notation'. A non-ASCII character is denoted by a leading '&#x', a trailing ';', and the hexadecimal number of the character in the UCS in between. For example, &#x42F; stands for CYRILLIC CAPITAL LETTER YA (Я). An actual '&' is denoted by '&amp;'. This notation is not used in the HTML and PDF versions of this document, because there, non-ASCII characters are used directly.

Where the IRI form of an example is identical to the URI form, only one form is given. If the IRI form is different, then both forms are given.

6.2.  Basic Examples

A URI for an ordinary individual mailing address:

<mailto:chris@example.com>

A URI for a mail response system that requires the name of the file to be sent back 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 URI, with 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 '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:

<mailto:list@example.org?In-Reply-To=%3C3469A91.D10AF4C@example.com%3E>

A request to subscribe to a mailing list:

<mailto:majordomo@example.com?body=subscribe%20bamboo-l>

A URI that is for a single user and that includes a CC of another user:

<mailto:joe@example.com?cc=bob@example.com&body=hello>

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

<mailto:joe@example.com?cc=bob@example.com?body=hello> ; WRONG!

According to [RFC5322] (Resnik, P., “Internet Message Format,” October 2008.), the characters "?", "&", and even "%" may occur in <addr-spec>s. 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 these cases.

To indicate the address "gorby%kremvax@example.com" one would use:

<mailto:gorby%25kremvax@example.com>

To indicate the address "unlikely?address@example.com", and include another header field, one would use:

<mailto:unlikely%3Faddress@example.com?blat=foop>

As described above, the "&" (ampersand) character is reserved in HTML and has to be replaced, e.g., with "&amp;". Thus, in an HTML context a URI with an internal ampersand might look like:

   Click  <a
   href="mailto:joe@an.example?cc=bob@an.example&amp;body=hello"
   >mailto:joe@an.example?cc=bob@an.example&amp;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 "Mike&family@example.org" is <mailto:Mike%26family@example.org>.

6.3.  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:

<mailto:%22not%40me%22@example.org>.

Email address: "oh\\no"@example.org; corresponding 'mailto' URI:

<mailto:%22oh%5C%5Cno%22@example.org>.

Email address: "\\\"it's\ ugly\\\""@example.org; corresponding 'mailto' URI:

<mailto:%22%5C%5C%5C%22it's%5C%20ugly%5C%5C%5C%22%22@example.org>.

6.4.  Examples Using UTF-8-Based Percent-Encoding usable with RFC 5322

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, as an URI:

<mailto:user@example.org?subject=caf%C3%A9>

The same as an IRI:

<mailto:user@example.org?subject=café>

The same subject, this time using an encoded-word (escaping the "=" and "?" characters used in the encoded-word syntax, because they are reserved):

<mailto:user@example.org?subject=%3D%3Futf-8%3FQ%3Fcaf%3DC3%3DA9%3F%3D>

The same subject, this time encoded as iso-8859-1:

<mailto:user@example.org?subject=%3D%3Fiso-8859-1%3FQ%3Fcaf%3DE9%3F%3D>

Going back to straight UTF-8 and adding a body with the same value, as an URI:

<mailto:user@example.org?subject=caf%C3%A9&body=caf%C3%A9>

The same as an IRI:

<mailto:user@example.org?subject=café&body=café>

This 'mailto' URI may result in an [RFC5322] (Resnik, P., “Internet Message Format,” October 2008.) message looking like this:

   From: sender@example.net
   To: user@example.org
   Subject: =?utf-8?Q?caf=C3=A9?=
   Content-Type: text/plain;charset=utf-8
   Content-Transfer-Encoding: quoted-printable

   caf=C3=A9

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:

   From: sender@example.net
   To: user@example.org
   Subject: =?iso-8859-1?Q?caf=E9?=
   Content-Type: text/plain;charset=iso-8859-1
   Content-Transfer-Encoding: quoted-printable

   caf=E9

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.

In a context where EAI is supported, this 'mailto' URI can result in an [RFC6532] (Yang, A., Steele, S., and N. Freed, “Internationalized Email Headers,” February 2012.) message looking like this (encoded as UTF-8 on the wire):

   From: sender@example.net
   To: user@example.org
   Subject: café
   Content-Type: text/plain;charset=utf-8
   Content-Transfer-Encoding: 8bit

   café

For more examples of encoding the word coffee in different languages, see [RFC2324] (Masinter, L., “Hyper Text Coffee Pot Control Protocol (HTCPCP/1.0),” April 1998.).

The following example uses the Japanese word "natto" (Unicode characters U+7D0D U+8C46, 納豆) as a domain name label, sending a mail to a user at 納豆.example.org, as an URI:

<mailto:user@%E7%B4%8D%E8%B1%86.example.org?subject=Test&body=%E7%B4%8D%E8%B1%86>

The same as an IRI:

<mailto:user@納豆.example.org?subject=Test&body=納豆>

When constructing the email for use with [RFC5322] (Resnik, P., “Internet Message Format,” October 2008.), the domain name label is converted to punycode. The resulting message might look as follows:

   From: sender@example.net
   To: user@xn--99zt52a.example.org
   Subject: Test
   Content-Type: text/plain;charset=utf-8
   Content-Transfer-Encoding: base64

   57SN6LGG

The same message using EAI ([RFC6532] (Yang, A., Steele, S., and N. Freed, “Internationalized Email Headers,” February 2012.)) can look as follows (encoded as UTF-8 on the wire):

   From: sender@example.net
   To: user@納豆.example.org
   Subject: Test
   Content-Type: text/plain;charset=utf-8
   Content-Transfer-Encoding: 8bit

   納豆

6.5.  Examples Using UTF-8-Based Percent-Encoding usable only with EAI

All the previous 'mailto' URIs can be used with EAI. When used with EAI, there is no need to use punycode in domain names, and no need to use MIME encoding in headers and bodies. After decoding percent-encoding, UTF-8 can be used directly. This subsection gives a few additional examples of 'mailto' URI and IRIs which can only be used with EAI.

Please note that the choice of URI vs. IRI is independent of whether EAI can be used or not.

A hypothetical 'mailto' URI for ordering coffee from a French coffee pot:

mailto:caf%C3%A9@pot.example?Subject=Espresso,%20please

The same as an IRI:

mailto:café@pot.example?Subject=Espresso,%20please

A hypothetical 'mailto' URI for sending a potential erratum to the first author of this memo ("%C3%BC" represents an u-umlaut, "%E9%9D%92%E5%B1%B1" represents the Unicode characters U+9752 (blue) and U+5C71 (mountain)):

mailto:Martin.D%C3%BCrst@%E9%9D%92%E5%B1%B1.example.net?Subject=Error%20in%20RFC6068bis

The same as an IRI:

mailto:Martin.Dürst@青山.example.net?Subject=Error%20in%20RFC6068bis

7.  Security Considerations

The 'mailto' URI/IRI 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/IRI 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/IRI, 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/IRI that would be used when clicked). Thus, a mail client SHOULD NOT send a message based on a 'mailto' URI/IRI 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/IRI), 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/IRI. Users are strongly encouraged to ensure that the 'mailto' URI/IRI 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/IRI. For details, please see Section 3 (Semantics and Operations). In general, the fewer header fields interpreted from the URI/IRI, 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 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/IRIs 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/IRIs on public Web pages expose mail addresses for harvesting. This applies to all mail addresses that are part of the 'mailto' URI/IRI, 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/IRI. 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/IRIs have to take great care to not inadvertently double-escape or double-unescape 'mailto' URIs/IRIs, and to make sure that escaping and unescaping conventions relating to URIs/IRIs and relating to mail addresses are applied in the right order.

Implementations parsing 'mailto' URIs/IRIs must take care to sanity check 'mailto' URIs/IRIs in order to avoid buffer overflows and problems resulting from them (e.g., execution of code specified by the attacker).

The security considerations of [STD66] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.), [RFC5890] (Klensin, J., “Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework,” August 2010.), [RFC5891] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” August 2010.), and [RFC3987] (Dürst, M. and M. Suignard, “Internationalized Resource Identifiers (IRIs),” January 2005.) also apply. Implementers and users are advised to check them carefully.

8.  IANA Considerations

8.1.  Update of the Registration of the 'mailto' URI/IRI Scheme

This document changes the definition of the 'mailto' URI/IRI scheme; the registry of URI/IRI schemes should be updated to refer to this document rather than its predecessor [RFC6068] (Dürst, M., Masinter, L., and J. Zawinski, “The 'mailto' URI Scheme,” October 2010.). The registration template is as follows:

URI scheme name:
   'mailto'

Status:
   permanent

URI/IRI scheme syntax:
   See the syntax section of RFC YYYY.
   [RFC Editor: Please replace with actual RFC number.]

URI/IRI scheme semantics:
   See the semantics section of RFC YYYY.
   [RFC Editor: Please replace with actual RFC number.]

Encoding considerations:
   See the syntax and encoding sections of RFC YYYY.
   [RFC Editor: Please replace with actual RFC number.]

Applications/protocols that use this URI scheme name:
   The 'mailto' URI/IRI scheme is widely used since
   the start of the Web.

Interoperability considerations:
   Interoperability for 'mailto' URIs/IRIs with UTF-8-based
   percent-encoding might be somewhat lower than interoperability
   for 'mailto' URIs with US-ASCII only. In particular,
   interoperability for 'mailto' URIs/IRIs with UTF-8-based
   percent-encoding in the LHS of email addresses requires
   support of EAI [RFC6530].

Security considerations:
   See the security considerations section of RFC YYYY.
   [RFC Editor: Please replace with actual RFC number.]

Contact:
   IETF

Author/Change controller:
   IETF

References:
   RFC YYYY  [RFC Editor: Please replace with actual RFC number.]

8.2.  Registration of the Body Header Field

IANA is herewith requested to update the reference for the registration of the Body header field in the Message Header Fields Registry ([RFC3864] (Klyne, G., Nottingham, M., and J. Mogul, “Registration Procedures for Message Header Fields,” September 2004.)) from [RFC6068] (Dürst, M., Masinter, L., and J. Zawinski, “The 'mailto' URI Scheme,” October 2010.) to this document (there are no changes to the specification of the Body header field itself).

9.  Main Changes from RFC 6068

The main changes from [RFC6068] (Dürst, M., Masinter, L., and J. Zawinski, “The 'mailto' URI Scheme,” October 2010.) are as follows:

• Allowed UTF-8/percent-encoding in <local-part-enc>, to be used for EAI email addresses.
• Added "/" and "?" back to some-delims, because they are allowed in query parts.
• Added suffix "-enc" to some ABNF rule names to distinguish them from their counterparts without percent-encoding.
• Added a MUST for using UTF-8 in <hfvalue>.
• Added examples as IRIs where there's a difference to the URI form.
• Added non-ASCII examples in HTML and PDF versions for better understanding.

10.  Change record

RFC Editor: Please remove this section before publication.

10.1.  Changes from -02 to -03

Introduced non-ASCII text in author names and examples for better understanding and as a trial for future draft/rfc formats.

Split "Main Changes" and changes by draft number so that the former can be kept, but the later removed when moving to publication.

Fixed title of RFC 6068.

Various minor tweaks and fixes.

10.2.  Changes from -01 to -02

TODO: Change syntax definition to be in terms of IRI syntax, not URI syntax.

Split up the Syntax section into subsections.

Added "/" and "?" back to some-delims, because they are allowed in query parts.

Updated references.

10.3.  Changes from -00 to -01

Updated references.

Removed RFC Editor note for updating reference to RFC3987. Depending on how the documents progress, this will be unnecessary or will happen automatically.

Minor editorial tweaks.

10.4.  Changes from RFC 6068 to -00

Changed title and various other places to also refer to IRIs.

Allowed UTF-8/percent-encoding in <local-part-enc>, to be used for EAI email addresses.

Updated syntax to use "-enc" prefix in some places.

Added MUST for using UTF-8 in <hfvalue>.

Added a new subsection with EAI-only examples.

Updated references.

Updated first author's address.

11.  Acknowledgments

This document was derived from [RFC6068] (Dürst, M., Masinter, L., and J. Zawinski, “The 'mailto' URI Scheme,” October 2010.); the acknowledgments from that specification and its predecessor still apply.

Valuable input on this document was received from (in no particular order): Shawn Steele, Frank Ellermann, John Klensin, and Yangwoo Ko (고양우).

12.  References

12.1. Normative References

12.2. Informative References

Authors' Addresses