MUA
A mail user agent
(MUA) is any of the many programs that users run to read, reply to, compose,
and dispose of email.
Examples of MUAs also
exist for PCs. Eudora and Claris-Works are two standalone MUAs. Netscape
and Explorer are web browsers that canalso act as MUAs. Thunderbird
is an open source MUA from the folks at Mozilla. Many MUAs can exist on a
single machine. MUAs sometimes perform limited mail transport, but this is
usually a very complex task for which they are not suited.
MTA
A mail transfer agent
(MTA) is a highly specialized program that delivers mail and transports it
between machines, like the post office does. Usually, there is only one MTA on
a machine. The sendmail program is an MTA.
MSA
sendmail also recognizes the
role of a mail submission agent (MSA), as defined in RFC2476. MTAs are not
supposed to alter an email’s text, except to add Received:, Return-Path:, and
other required headers. Email submitted by an MUA might require more
modification than is legal for an MTA to perform, so the new role of an MSA was
created. An MSA accepts messages from an MUA, and has the legal right to
heavily add to, subtract from, and screen or alter all such email. An MSA, for
example, can ensure that all hostnames are fully qualified, and that headers,
such as Date:, are always included.
