Greylisting

Greylisting is a great way to keep spam and viruses out. Unknown senders will be rejected with a temporary error message. At the second attempt (mail servers try to deliver their mail more than once) the sender address is known (the send IP, sender and receiver address, to be precice) and will be accepted without delay.

Viruses and spammers will not try a second time, so that crap wont reach the server.

See statistics for the first couple of days