Short for message transfer agent, the program responsible for
receiving incoming e-mails and delivering the messages to
individual users. The MTA transfers messages between computers.
Hidden from the average user, it is responsible for routing
messages to their proper destinations. MTAs receive messages
from both MUAs and other MTAs, although single-user machines
more often retrieve mail messages using POP. The MTA is commonly
referred to as the mail server program. UNIX sendmail and
Microsoft Exchange Sever are two examples of MTAs.