How greylisting can reduce the spam.(Brief article)

Byline: Nigel Whitfield

How greylisting can reduce the spam

One technique that many mail administrators are using is greylisting. So far, it's proving surprisingly effective at rejecting a lot of spam and, unlike other solutions, there's no risk of false positives and false negatives.

Greylisting (www.greylisting.org) doesn't worry about the content of an email. Instead, when an email server receives a message, it simply rejects it with a temporary failure code. A standard mail server will see the response, and simply queue the message to deliver later. Most spam software doesn't do that; it just sees anything other than an 'OK' response as a problem, and goes on to the next address on the list.

Genuine messages will be retried by the sending server -- how often depends on the way they're configured, but half an hour or an hour is common. The next time the greylisting system sees the same combination of sending server, address and recipient, it passes the mail through ...