Unicode and email
|This article does not cite any references or sources. (February 2013)|
Many email clients now offer some support for Unicode.  as the reader client might not support it, but as time passes, more and more systems are likely to be set up with fonts capable of displaying the full range of Unicode characters (or at least the set likely to be of interest to the user).
Technical requirements for sending of messages containing non-ASCII characters by email include
- encoding of certain headings (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
- 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 message headings
To use Unicode in certain email headings, eg 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 and email addresses, IDNA must be used. Various standards had been created to retrofit the handling of non-ASCII data to the originally ASCII-only email protocol:
- RFC 2047 provides support for encoding non-ASCII values such as real names and subject lines in email headers
- RFC 3490 provides support for encoding non-ASCII domain names in the Domain Name System
- RFC 6531 provides support for encoding non-ASCII names in the local part and transporting these via SMTP
Unicode support in message bodies
UTF-7, although sometimes considered deprecated, has an advantage over other Unicode encodings in that it does not require a transfer encoding to fit within the seven-bit limits of many[quantify] legacy Internet mail servers. On the other hand, UTF-16 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) or, for some languages and types of text, heavily size inefficient (in the case of quoted-printable).
Some document formats, such as HTML, PostScript and Rich Text Format 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 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. The rest of this article[specify] deals with email messages where the actual raw text (whether markup or plain text) is in an encoding that covers the whole of Unicode.