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The Protocol used in the mail send / retrieve process：
1. Send E-Mail：It means the process of users send the mails to MTA via MUA and transfer the mails to the
next MTA. Most of the mail server use the SMTP Protocol ( Simple Mail Transfer Protocol ) ，and the port
number is 25 .
2. Retrieve E-Mail：It means the recipient use MUA to link to MTA mailbox through the POP Protocol ( Post
Office Protocol ) ，in order to read or download the mails from mailbox . The most common used POP
Protocol is POP3 ( Post Office Protocol version 3 )，and its port number is 110 .
Basically, the MTA need at least two Protocols, and it included SMTP and POP3. Both of the MUA and
MTA can communicate to each other only if the MUA and MTA support SMTP and POP3.
The MTA can analyze the received mails to check if the recipient mail account existed in MTA or it will
relay the mails to the next MTA.
The Open Relay mail server can allow anyone to use one of the mail server to send the mails. To avoid
this problem, most of the mail server setting still disable the relay function. The Mail Server only enable the
relay function to Local host, so that the MTA can receive the mails from the network with the recipient mail
account existed in MTA server. Basically, there is no big problem with MTA mail Retrieve function except one
condition. Normally, the MTA only enable the relay function to some local host with regulated IP and domain.
The Client PC can freely send / retrieve the mails. In other words, the sender mail will be blocked if it is not in
the regulated range. We can use the SMTP to solve the problem.
The so called SMTP is that the MTA server will require to check the MUA User's Mail Account and
Password, then MTA will provide the relay function to the authenticated user without regulating the IP and
domain. It is because the MTA can analyze the sender Authentication information via Authentication function.
After the authentication, MTA will continue to relay the sender mails.
CS-2000 UTM Content Security Gateway User's Manual
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