The collection, sorting, transport, and delivery of e-mail
over a network is handled by two utilities:
MUA
The Mail User Agent is the program that you use to create, send,
read, and organize your messages.
SCO systems provide the choice of several MUAs:
mail,
SCO Shell Mail,
and
scomail(XC).
MTA
The Mail Transport Agent is the program that transports mail,
puts the messages you send in the outgoing mail queue,
and reads messages from the incoming queue to deliver to
your mailbox.