| Greylisting. Whats that? |
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In the ongoing battle againest spam and spammers a clever fellow by the name of Evan Harris proposed a method to reject much spam by exploiting a weakness in the spammers delivery system.
In the event that a message fails to be delivered a legitimate mail server will resend the message (and continue to do so for several days until it times out). This mechanism ensures that temporary network outages do not result in loss of the mesaage. Most spam is not delivered by legitimate servers and , in the event of non delivery, the message is not resent.
Greylisting works by simply temporarily rejecting all mail, keeping a note of the messages details (the grey list) and then waiting to see if the message is delivered again, if it is it will be accepted for further processing. Most Greylisting programs offer more advanced filtering features than this basic technique and are extremely effective at excluding spam from the MTA (Mail Transport Agent). |
