http://www.ietf.org/rfc/rfc2476.txt
SMTP was defined as a message *transfer* protocol, that is, a means to route (if needed) and deliver finished (complete) messages. Message Transfer Agents (MTAs) are not supposed to alter the message text, except to add 'Received', 'Return-Path', and other header fields as required by [SMTP-MTA].

However, SMTP is now also widely used as a message *submission* protocol, that is, a means for message user agents (MUAs) to introduce new messages into the MTA routing network. The process which accepts message submissions from MUAs is termed a Message Submission Agent (MSA).

Messages being submitted are in some cases finished (complete) messages, and in other cases are unfinished (incomplete) in some aspect or other. Unfinished messages need to be completed to ensure they conform to [MESSAGE-FORMAT], and later requirements. For example, the message may lack a proper 'Date' header field, and domains might not be fully qualified. In some cases, the MUA may be unable to generate finished messages (for example, it might not know its time zone). Even when submitted messages are complete, local site policy may dictate that the message text be examined or modified in some way. Such completions or modifications have been shown to cause harm when performed by downstream MTAs -- that is, MTAs after the first-hop submission MTA -- and are in general considered to be outside the province of standardized MTA functionality.

Separating messages into submissions and transfers allows developers and network administrators to more easily:

• Implement security policies and guard against unauthorized mail relaying or injection of unsolicited bulk mail
• Implement authenticated submission, including off-site submission by authorized users such as travelers
• Separate the relevant software code differences, thereby making each code base more straightforward and allowing for different programs for relay and submission
• Detect configuration problems with a site's mail clients
• Provide a basis for adding enhanced submission services in the future

This memo describes a low cost, deterministic means for messages to be identified as submissions, and specifies what actions are to be taken by a submission server.