PLANET CS-2001 - Manual (Page 466)

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The Three Key Elements of Email Transmission
An email transmission is achieved by using an MUA, MTA and MDA.

MUA（Mail User Agent） ：Whether sending or receiving email, the end-user
client must rely on an MUA which came along with the OS, as without it they
are unable to obtain email access. Outlook Express, for example, serves as a mail
deliverer and recipient, as well as enables user to view and compose emails.

MTA（Mail Transfer Agent） ：Email delivery and retrieval are done by a MTA.
It allows users to:
 Receive emails from other hosts. As long as user has an account on the
MTA, then the user will be able to receive emails.
 Sent out emails. If the user is authorized, he or she will be able to send out
emails.
 Retrieve emails from the mail server.

MDA（Mail Delivery Agent） ：It distributes incoming emails from the MTA to
local email accounts based on the account names or passes them on to the next
MTA.
Note：
1. Usually, MTA refers to an mail server.