PLANET CS-2001 - Manual (Page 468)

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 Email retrieval: signifies MUA is using POP (Post Office Protocol) to communicate with
the MTA by which users may have the access to emails. Currently, POP3 (Post Office
Protocol version 3) is the most popular protocol for incoming emails. By default, port 110
is assigned to the POP3 protocol.
2. MTA needs at least two protocols, namely SMTP and POP3, to receive and send emails. As
long as both your MUA and MTA support SMTP and POP3 protocols, you will be able to
have email access.
3. Relay, a function that MTA transfers emails to another MTA, functions when MTA has
received emails intended for external recipients, it will pass them onto the destination MTA.
4. When a mail server can be used for sending emails by anyone, it is called “Open Relay”. To
avoid this, nowadays most mail servers preserve the “Relay” functionality to the local MTA.
Only the local MTA can utilize relay to receive emails that were specified for its internal
accounts. Nevertheless, there is a drawback in using relay. Since relay normally only allows
email from certain trusted mail servers with specified IPs or segments, emails from other
unspecified mail servers will be rejected. SMTP authentication, therefore, is the solution to
this situation.
5. SMTP authentication, a mechanism whereby the MTA inspects the sender’s address and
password on the MUA, takes place when emails transfer from the MUA to the MTA. It has a
benefit of enabling relay only to the authorized accounts rather than just certain trusted
domain names or IP addresses. Whenever SMTP authentication is employed, the MTA will
neither accept incoming emails nor provide email delivery services unless the sender is
authenticated.