Mail Relaying and Spam Control
You can't have a mail server online on the Internet without it being probed by spammers to see if they can use it for broadcasting their junk mail all over the world—and if they can, they will. Relaying mail means passing on messages that neither originate from nor end up on the local system. That's what most spammers do: They connect from a remote, anonymous location to any “open” mail server that they can find that imposes no restrictions on who can connect to them and relay mail through to other servers.
For a legitimate dial-up or remote user to use your SMTP server to send a message to another remote recipient, your server has to act as a relay, forwarding the message to the recipient even if the message didn't ...
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