> So back to my question, why do mail servers relay mail?
There are two reasons, depending on how you define "relay".
The true meaning of relay, when dealing with mail servers, is to accept an E-mail, and
pass it on to another mail server that will accept it and take care of delivering it.
You need to be able to relay for your users. They connect to your mail server, which
accepts the E-mail, and sends it to another mail server (based on the MX record for
the recipient's domain). If E-mail clients delivered directly to the recipient's mail
server, relaying wouldn't be needed. But, E-mail clients use a "smart host" (your
mail server) in order to deliver the mail. That's why relaying is necessary.
The other meaning of relay is "unauthorized relaying", which today almost always means
spammers hijacking your mail server. Back in 1982 when the first SMTP mail servers
were written, they were intentionally designed specifically to allow mail from any
server to any server. There were a couple of reasons for this. First, the delivery
mechanism was more complex, and it might be that you would have to deliver to a third
party's mail server to get your mail to the final destination. Also, the Internet was
designed to be "friendly" (for lack of a better word) and even though it was more
expensive to maintain back then, people were more well behaved.
Between 1982 and the early 1990s, the first spam started arriving. Shortly
thereafter, the less ethical spammers found that it was more convenient to use someone
else's mail server to send their mail, as they would be less likely to get caught, and
could send out more mail more quickly. Finally, a couple years ago, administrators
starting catching on that the old "relay for anyone" concept was now "relay for
spammers". Almost nobody was relaying through third party mail servers, except for
spammers. So, the rules changed (there's even an RFC now suggesting -- but not
requiring -- people to close open relays), but it takes years on the Internet for
change in software to become widespread.
--
-Scott
Declude: Anti-virus and Anti-spam solutions for IMail. http://www.declude.com
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