Internationalized Email Headers
Full internationalization of electronic mail requires not only the capability to transmit non-ASCII content, to encode selected information in specific header fields, and to use non-ASCII characters in envelope addresses. It also requires being able to express those addresses and information based on them in mail header fields. This document specifies the use of Unicode encoded in UTF-8, rather than ASCII, as the base form for Internet email header field bodies. This form is permitted in transmission only if authorized by an SMTP extension, as specified in an associated specification.