Unicode and email

Support in protocols

provides a mechanism for allowing non-ASCII email addresses encoded as UTF-8 in the SMTP and LMTP protocols.

Message headers

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 the 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.

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, an obsolete encoding, had an advantage on obsolete non–8-bit clean networks over Unicode encodings in that it does not require a transfer encoding to fit within the 7-bit limits of legacy Internet mail servers. On the other hand, UTF-16 must be transfer encoded to fit the data format of SMTP. Although not strictly required, UTF-8 is usually also transfer encoded to avoid problems across 7-bit mail servers. MIME transfer encoding of UTF-8 makes it either unreadable as 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. 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).

References

References

1. "wanderlust/apel".
2. "Setting Outlook to Use UTF-8".
3. (February 2012). "SMTP Extension for Internationalized Email".
4. Moore, Keith. (November 1996). "MIME (Multipurpose Internet Mail Extensions) Part Three: Message Header Extensions for Non-ASCII Text".
5. Klensin, John C. (August 2010). "Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework".
6. (February 2012). "Internationalized Email Headers".