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email client An email client, email reader or, more formally, message user agent (MUA) or mail user agent is a computer program used to access and manage a user's email. A web application which provides message management, composition, and reception functio ...
s now offer some support for Unicode. Some clients will automatically choose between a legacy encoding and Unicode depending on the mail's content, either automatically or when the user requests it. Technical requirements for sending of messages containing non-
ASCII ASCII ( ), abbreviated from American Standard Code for Information Interchange, is a character encoding standard for electronic communication. ASCII codes represent text in computers, telecommunications equipment, and other devices. Because of ...
characters by email include * encoding of certain header fields (subject, sender's and recipient's names, sender's organization and reply-to name) and, optionally, body in a content-transfer encoding * encoding of non-ASCII characters in one of the Unicode transforms * negotiating the use of UTF-8 encoding in email addresses and reply codes ( SMTPUTF8) * sending the information about the content-transfer encoding and the Unicode transform used so that the message can be correctly displayed by the recipient (see Mojibake). If the sender's or recipient's email address contains non-ASCII characters, sending of a message requires also encoding of these to a format that can be understood by mail servers.


Unicode support in protocols

* provides a mechanism for allowing non-ASCII email addresses encoded as UTF-8 in an
SMTP The Simple Mail Transfer Protocol (SMTP) is an Internet standard communication protocol for electronic mail transmission. Mail servers and other message transfer agents use SMTP to send and receive mail messages. User-level email clients typica ...
or
LMTP The Local Mail Transfer Protocol (LMTP) is an alternative to (Extended) Simple Mail Transfer Protocol for situations where the receiving side does not have a mail queue, such as a message transfer agent acting as a message delivery agent. LMTP was ...
protocol


Unicode support in message header

To use Unicode in certain email header fields, e.g. subject lines, sender and recipient names, the Unicode text has to be encoded using a MIME "Encoded-Word" with a Unicode encoding as the charset. To use Unicode in domain part of email addresses, IDNA encoding must traditionally be used. Alternatively, SMTPUTF8 allows the use of UTF-8 encoding in email addresses (both in a local part and in domain name) as well as in a mail header section. Various standards had been created to retrofit the handling of non-ASCII data to the originally ASCII-only email protocol: * provides support for encoding non-ASCII values such as real names and subject lines in email headers * provides support for encoding non-ASCII domain names in the Domain Name System * allows the use of UTF-8 in a mail header section


Unicode support in message bodies

As with all encodings apart from US-ASCII, when using Unicode text in email, MIME must be used to specify that a Unicode transformation format is being used for the text.
UTF-7 UTF-7 (7- bit Unicode Transformation Format) is an obsolete variable-length character encoding for representing Unicode text using a stream of ASCII characters. It was originally intended to provide a means of encoding Unicode text for use in In ...
, an obsolete encoding, had an advantage over Unicode encodings, on obsolete non-8bit-clean networks, in that it does not require a transfer encoding to fit within the seven-bit limits of legacy Internet mail servers. On the other hand,
UTF-16 UTF-16 (16-bit Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 valid code points of Unicode (in fact this number of code points is dictated by the design of UTF-16). The encoding is variable-length, as cod ...
must be transfer encoded to fit SMTP data format. Although not strictly required, UTF-8 is usually also transfer encoded to avoid problems across seven-bit mail servers. MIME transfer encoding of UTF-8 makes it either unreadable as a plain text (in the case of
base64 In computer programming, Base64 is a group of binary-to-text encoding schemes that represent binary data (more specifically, a sequence of 8-bit bytes) in sequences of 24 bits that can be represented by four 6-bit Base64 digits. Common to all bi ...
) or, for some languages and types of text, heavily size inefficient (in the case of quoted-printable). Some document formats, such as HTML,
PostScript PostScript (PS) is a page description language in the electronic publishing and desktop publishing realm. It is a dynamically typed, concatenative programming language. It was created at Adobe Systems by John Warnock, Charles Geschke, Doug B ...
and
Rich Text Format ) As an example, the following RTF code would be rendered as follows: This is some bold text. Character encoding A standard RTF file can only consist of 7-bit ASCII characters, but can use escape sequences to encode other characters. Th ...
have their own 7-bit encoding schemes for non-ASCII characters and can thus be sent without using any special email encodings. E.g. HTML email can use
HTML entities In SGML, HTML and XML documents, the logical constructs known as ''character data'' and ''attribute values'' consist of sequences of characters, in which each character can manifest directly (representing itself), or can be represented by a series ...
to use characters from anywhere in Unicode even if the HTML source text for the email is in a legacy encoding (e.g. 7-bit ASCII). For details of this see
Unicode and HTML Web pages authored using HyperText Markup Language (HTML) may contain multilingual text represented with the Unicode universal character set. Key to the relationship between Unicode and HTML is the relationship between the "document character se ...
.


See also

*
Comparison of email clients The following tables compare general and technical features of notable email client programs. General Basic general information about the clients: creator/company, O/S, licence, & interface. Clients listed on a light purple background are no long ...
* International email


References


External links


SIL's freeware fonts, editors and documentation
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