1. Introduction
1.1. Scope
This standard specifies a syntax for text messages that are sent between computer users, within the framework of "electronic mail" messages. This standard supersedes the one specified in Request For Comments (RFC) 822std11(-> 2822prop), "Standard for the Format of ARPA Internet Text Messages" [RFC822], updating it to reflect current practice and incorporating incremental changes that were specified in other RFCs [STD3].
This standard specifies a syntax only for text messages. In particular, it makes no provision for the transmission of images, audio, or other sorts of structured data in electronic mail messages. There are several extensions published, such as the MIME document series [RFC2045, RFC2046, RFC2049], which describe mechanisms for the transmission of such data through electronic mail, either by extending the syntax provided here or by structuring such messages to conform to this syntax. Those mechanisms are outside of the scope of this standard.
In the context of electronic mail, messages are viewed as having an envelope and contents. The envelope contains whatever information is needed to accomplish transmission and delivery. (See [RFC2821] for a discussion of the envelope.) The contents comprise the object to be delivered to the recipient. This standard applies only to the format and some of the semantics of message contents. It contains no specification of the information in the envelope.
However, some message systems may use information from the contents to create the envelope. It is intended that this standard facilitate the acquisition of such information by programs.
This specification is intended as a definition of what message content format is to be passed between systems. Though some message systems locally store messages in this format (which eliminates the need for translation between formats) and others use formats that differ from the one specified in this standard, local storage is outside of the scope of this standard.
Note: This standard is not intended to dictate the internal formats used by sites, the specific message system features that they are expected to support, or any of the characteristics of user interface programs that create or read messages. In addition, this standard does not specify an encoding of the characters for either transport or storage; that is, it does not specify the number of bits used or how those bits are specifically transferred over the wire or stored on disk.
1.2. Notational conventions
1.2.1. Requirements notation
This document occasionally uses terms that appear in capital letters. When the terms "MUST", "SHOULD", "RECOMMENDED", "MUST NOT", "SHOULD NOT", and "MAY" appear capitalized, they are being used to indicate particular requirements of this specification. A discussion of the meanings of these terms appears in [RFC2119].
1.2.2. Syntactic notation
This standard uses the Augmented Backus-Naur Form (ABNF) notation specified in [RFC2234] for the formal definitions of the syntax of messages. Characters will be specified either by a decimal value (e.g., the value %d65 for uppercase A and %d97 for lowercase A) or by a case-insensitive literal value enclosed in quotation marks (e.g., "A" for either uppercase or lowercase A). See [RFC2234] for the full description of the notation.
1.3. Structure of this document
This document is divided into several sections.
This section, section 1, is a short introduction to the document.
Section 2 lays out the general description of a message and its constituent parts. This is an overview to help the reader understand some of the general principles used in the later portions of this document. Any examples in this section MUST NOT be taken as specification of the formal syntax of any part of a message.
Section 3 specifies formal ABNF rules for the structure of each part of a message (the syntax) and describes the relationship between those parts and their meaning in the context of a message (the semantics). That is, it describes the actual rules for the structure of each part of a message (the syntax) as well as a description of the parts and instructions on how they ought to be interpreted (the semantics). This includes analysis of the syntax and semantics of subparts of messages that have specific structure. The syntax included in section 3 represents messages as they MUST be created. There are also notes in section 3 to indicate if any of the options specified in the syntax SHOULD be used over any of the others.
Both sections 2 and 3 describe messages that are legal to generate for purposes of this standard.
Section 4 of this document specifies an "obsolete" syntax. There are references in section 3 to these obsolete syntactic elements. The rules of the obsolete syntax are elements that have appeared in earlier revisions of this standard or have previously been widely used in Internet messages. As such, these elements MUST be interpreted by parsers of messages in order to be conformant to this standard. However, since items in this syntax have been determined to be non-interoperable or to cause significant problems for recipients of messages, they MUST NOT be generated by creators of conformant messages.
Section 5 details security considerations to take into account when implementing this standard.
Section 6 is a bibliography of references in this document.
Section 7 contains the editor's address.
Section 8 contains acknowledgements.
Appendix A lists examples of different sorts of messages. These examples are not exhaustive of the types of messages that appear on the Internet, but give a broad overview of certain syntactic forms.
Appendix B lists the differences between this standard and earlier standards for Internet messages.
Appendix C has copyright and intellectual property notices.