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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.