RFC 2476 Definitions

By the way, here are the definitions from RFC 2476:

    Message Submission Agent (MSA)

    A process which conforms to this specification, which 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 which conforms to [SMTP-MTA], which 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 which acts (usually on behalf of a user) to compose and
    submit new messages, and process delivered messages.  In the split-
    MUA model, POP or IMAP is used to access delivered messages.

Note that an MSA "accepts" messages from an MUA, so in the case that 
was mentioned where a process invokes sendmail, it can be said that 
sendmail accepts the message from the process.

-- 
Randall Gellens
Opinions are personal;    facts are suspect;    I speak for myself only
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An algorithm must be seen to be believed.  -- Donald Knuth