AirLive ES-6000 - Manual (Page 221)

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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.