AirLive RS-3000 - Manual (Page 165)

2. Receiving e-mail: MUA connect to MTA user’s Mailbox by POP (Post Office Protocol) in order to
read or download the mail in user’s mailbox. At present, common POP Protocol is POP3 (Post
Office Protocol version 3), and the Port Number is 110.
Generally, a MTA that provides sending/receiving mail function needs two protocols at least. They
are SMTP and POP3. And as long as your MUA and MTA support SMPT and POP3, then they can
connect with each other.
After MTA analyzing the received mail and if the recipient is not in the master account, then MTA
will transfer the mail to the next MTA. This function is called Relay.
If anyone can deliver the mail by one of the mail server, we called this Open Relay mail server.
To avoid this question, most of the mail server’s default value will not open up Relay function. It only will
open up Relay function according to Localhost. Therefore, MTA can receive the mail that indicative of
the recipient is the internal account of MTA mail server. So there is no problem in receiving the mail.
However it causes some problems because MTA only setup some standard IP and Subnet to open their
Relay function. So in the range of this setting, the Client can send/receive mail very free. As for the mail
from the IP source without standard will be blocked completely. In this case, there comes Simple Mail
Transfer Protocol to solve the problem.
Simple Mail Transfer Protocol is when MUA send mail to MTA; the master will ask to detect the
account and password of MUA sender. And then MTA can provide the Relay function after
authentication without setup Relay function according to some trusting domain or IP. By Authentication,
MTA will analyze the relevant authentication information of the sender. After passing the authentication
that will accept mail and send the mail, otherwise; MTA will not receive the mail.
163