[2] Technical requirements for sending of messages containing non-ASCII characters by email include 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.
To use Unicode in the domain part of email addresses, IDNA encoding must traditionally be used.
Alternatively, SMTPUTF8[3] 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: 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.
Although not strictly required, UTF-8 is usually also transfer encoded to avoid problems across seven-bit mail servers.