2. Document Information

2.1. Definitions of Terms Used in This Memo

Many of the concepts and terms used in this document are defined in [SMTP-MTA]; familiarity with those documents is assumed here.

Fully-Qualified

Containing or consisting of a domain that can be globally resolved using the Domain Name Service; that is, not a local alias or partial specification.

Message Submission Agent (MSA)

A process that conforms to this specification. An MSA acts as a submission server to accept messages from MUAs, and either delivers them or acts as an SMTP client to relay them to an MTA.

Message Transfer Agent (MTA)

A process that conforms to [SMTP-MTA]. An MTA acts as an SMTP server to accept messages from an MSA or another MTA, and either delivers them or acts as an SMTP client to relay them to another MTA.

Message User Agent (MUA)

A process that acts (often on behalf of a user and with a user interface) to compose and submit new messages, and process delivered messages.

For delivered messages, the receiving MUA may obtain and process the message according to local conventions or, in what is commonly referred to as a split-MUA model, Post Office Protocol [POP3] or IMAP [IMAP4] is used to access delivered messages, whereas the protocol defined here (or SMTP) is used to submit messages.

2.2. Conventions Used in This Document

In examples, "C:" is used to indicate lines sent by the client, and "S:" indicates those sent by the server. Line breaks within a command example are for editorial purposes only.

Examples use the 'example.net' domain.

The key words "MUST", "MUST NOT", "SHOULD", "SHOULD NOT", and "MAY" in this document are to be interpreted as defined in [KEYWORDS].