Greylisting

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Greylisting is a simple method of defending electronic mail users against e-mail spam. It is the best way of blocking most spam without using content filters (which can cause false positives). So with this option we do not have any false positives (so no email will be lost) but we also do not act as a spammers paradise to spam you.

In short, a mail transfer agent which uses greylisting will "temporarily reject" (a 450 error code) any email from a sender it does not recognize. If the mail is legitimate, the originating server will try again to send it later (within a few minutes), at which time the destination will accept it. If the mail is from a spammer, it will probably not be retried so the spam will not be relayed through our sytems into your mail server.

If you tested the servers first you may have noticed the "450" code saying to try again (it was not a bounce). But if you try to test the servers a few minutes later you will see a success message. Greylisting is nice because all mail servers will retry within a few minutes and this time the mail will be accepted.