ARTICLE TO BE COMPLETED

Geylisting is a technique used to block bad mail clients of the sort used as spambots before they have a chance to dump their load into our mail server.
Whenever a mail server connects to our server to deliver mail we will initially refuse the connection, asking them to retry a few minutes later. A normally behaved mail server will systematically retry. Postgrey will recognized the second attempt and will let it through. Most spammers don't have the luxury of keeping track of all the emails they have to resend and just move on to their next target. This is because sending email takes resources, and delaying and resending takes more resources than it's worth to them: their objectives is to dump as much spam as quickly as possible before their become blacklisted. Their window of opportunity is quite narrow.

Greylisting can be configured to let through some recipients or some servers that we know are friendly. At any rate you should not lose any mail because of greylisting: any mail sent through a RFC compliant (normal) mail server will end up being delivered.
The only downside to greylisting is that it introduces a short few minutes delay in mail delivery. I don't know about you, but given the recent spam increase, I can live with that!

Getting postgrey There are ready-made packages for most distributions

http://www.greylisting.org/implementations/