2. Terminology Used in This Specification

A plain ASCII string is fully compatible with [RFC5321] and [RFC5322]. In this document, non-ASCII strings are UTF-8 strings if they are in header field values that contain at least one <UTF8-non-ascii> (see Section 3.1).

Unless otherwise noted, all terms used here are defined in [RFC5321], [RFC5322], [RFC6530], or [RFC6531].

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 [RFC2119].

The term "8-bit" means octets are present in the data with values above 0x7F.