AirLive ES-4000 v2 - Manual (Page 168)

Relay, a function that MTA transfers mails to another MTA, functions when MTA has learned that mail
recipient does not exist on itself will transfer mails to another MTA.
A mail server which could be used to transfer mails by anyone is called Open Relay. To avoid this,
presently most of mail servers set Relay as disabled. Only local host can utilize relay to receive mails that
specified for this MTA accounts form Internet. Basically, there is no doubt that relay enables MTA to receive
mails, yet there is a drawback in it. Since MTA normally allows certain trusted mail servers with specified IPs
and segments to relay, so that client user could use mail services without limitation while other unspecified
users been blocked. Thus, SMTP Authentication is invented to solve this problem.
SMTP Authentication, a mechanism that MTA inspects sender account and password from MUA,
happens when mails transfer from MUA to MTA. It has a benefit of enabling relay function to authentic MTA
accounts rather than certain trusted domain name or IP. Because of SMTP Authentication, MTA will neither
accept incoming mails nor provide mail delivery service unless the sender is authentic.
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