1. Introduction

Internet mail distinguishes a message from its transport and further divides a message between a header and a body [RFC5322]. Internet mail header field values contain a variety of strings that are intended to be user-visible. The range of supported characters for these strings was originally limited to [ASCII] in 7-bit form. MIME [RFC2045] [RFC2046] [RFC2047] provides the ability to use additional character sets, but this support is limited to body part data and to special encoded-word constructs that were only allowed in a limited number of places in header field values.

Globalization of the Internet requires support of the much larger set of characters provided by Unicode [RFC5198] in both mail addresses and most header field values. Additionally, complex encoding schemes like encoded-words introduce inefficiencies as well as significant opportunities for processing errors. And finally, native support for the UTF-8 charset is now available on most systems. Hence, it is strongly desirable for Internet mail to support UTF-8 [RFC3629] directly.

This document specifies an enhancement to the Internet Message Format [RFC5322] and to MIME that permits the direct use of UTF-8, rather than only ASCII, in header field values, including mail addresses. A new media type, message/global, is defined for messages that use this extended format. This specification also lifts the MIME restriction on having non-identity content-transfer-encodings on any subtype of the message top-level type so that message/global parts can be safely transmitted across existing mail infrastructure.

This specification is based on a model of native, end-to-end support for UTF-8, which depends on having an "8-bit-clean" environment assured by the transport system. Support for carriage across legacy, 7-bit infrastructure and for processing by 7-bit receivers requires additional mechanisms that are not provided by these specifications.

This specification is a revision of and replacement for [RFC5335]. Section 6 of [RFC6530] describes the change in approach between this specification and the previous version.