Many e-Mail clients now offer some support for Unicode in e-mail bodies. Most do not send in Unicode by default, 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).

To use Unicode in e-mail subject lines and e-mail addresses two different standards need to be used to retrofit the handling of non-ASCII data to the originally ASCII-only e-mail protocol:

• RFC 2047 provides support for encoding non-ASCII values such as real names and subject lines in e-mail headers
• RFC 3490 provides support for encoding non-ASCII domain names in the Domain Name System

Unicode support in message bodies

As with all encodings apart from US-ASCII, when using Unicode text in e-mail, MIME must be used to specify that a Unicode transformation format is being used for the text. To use Unicode in e-mail headers, the Unicode text has to be encoded using a MIME "Encoded-Word" with a Unicode encoding as the charset.

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 legacy Internet mail servers. UTF-8 and UTF-16 on the other hand must be transfer encoded in base64 or quoted-printable to allow safe transmission across seven-bit mail servers (i.e., those that do not advertise 8BITMIME).

Some document formats, such as HTML, PostScript and Rich Text Format can use 7 bit codes for Unicode characters and can thus be sent without using any special e-mail encodings. E.g. HTML e-mail can use HTML entities to use characters from anywhere in Unicode even if the HTML source text for the e-mail is in a legacy encoding (e.g. 7-bit ASCII). For details of this see Unicode and HTML. The rest of this article deals with e-mail messages where the actual raw text (whether markup or plain text) is in an encoding that covers the whole of Unicode.