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MUA: (Mail User Agent) Used for reading, writing and replying to mail. The MUA hands the mail to an MTA.
MTA: (Mail Transport Agent) Transports mail between systems. It is responsible for seeing that a piece of mail is transported to the correct destination machine. If you hand a piece of mail to an MTA, the MTA will see that the mail gets to the correct host in the address. The host is then responsibile for seeing that the user actually gets the mail by using the MDA.
MDA: (Mail Delivery Agent) Delivers mail to the mailboxes of users on the system the agent runs on. Usually, mail is handed to the MDA by the MTA on the same system. Sendmail and smail are both an MTA and an MDA, but in our examples we use them only as an MTA.
mailbox/folder: Where the mail is stored by the MDA. The MUA looks here for mail for you to read.
POP: Stands for post office protocol. Currently, most ISPs use version 2 (POP2), version 3(POP3) or APOP (POP3 with MDS authentication), although some may use RPOP (POP3 with RPOP authentication) or KPOP (POP3 with Kerberos authentication).