What is greylisting?

The term greylisting (British) or graylisting (U.S.) refers to a form of spam prevention for e-mails in which the first e-mail from unknown senders is initially rejected and only accepted after a further delivery attempt.

If it is a legitimate message, the server that sent the e-mail tries to send it again after a short pause. This is a standard behavior for e-mail transport protocols.

If the message is resent by the sending server after some time, the receiving server will have the data in its greylist database and will accept the message.

The message is slightly delayed by greylisting, but the risk of spam is lower, since most spammers do not resend SPAM messages that were initially rejected.

Mailix does not use Greylistung in general, but only for e-mail messages above a certain SPAM value.

See also:
Mailix Documentation: What is “greylisting”?
Wikipedia: Greylisting