1. Introduction

In order to use internationalized email addresses, it is necessary to internationalize both the domain part and the local part of email addresses. The domain part of email addresses is already internationalized [RFC5890], while the local part is not. Without the extensions specified in this document, the mailbox name is restricted to a subset of 7-bit ASCII [RFC5321]. Though MIME [RFC2045] enables the transport of non-ASCII data, it does not provide a mechanism for internationalized email addresses. In RFC 2047 [RFC2047], MIME defines an encoding mechanism for some specific message header fields to accommodate non-ASCII data. However, it does not permit the use of email addresses that include non-ASCII characters. Without the extensions defined here, or some equivalent set, the only way to incorporate non-ASCII characters in any part of email addresses is to use RFC 2047 coding to embed them in what RFC 5322 [RFC5322] calls the "display name" (known as a "name phrase" or by other terms elsewhere) of the relevant header fields. Information coded into the display name is invisible in the message envelope and, for many purposes, is not part of the address at all.

This document is a replacement for RFC 4952 [RFC4952]; it reflects additional issues, shared terminology, and some architectural changes identified since that document was published. It obsoletes that document. The experimental descriptions of in-transit downgrading [RFC5504] [RFC5825] are now irrelevant and no longer needed due to the changes discussed in Section 12. The RFC Editor is requested to move all three of those documents to Historic.

The pronouns "he" and "she" are used interchangeably to indicate a human of indeterminate gender.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [RFC2119].