Paul McCormack’s Web Site

Postgrey is a very simple, but apparently effective spam fighting tool that looks at an incoming mail message as it is sent to the server. Before accepting the full mail, Postgrey checks to see if the combination of the source, sender and recipient are recently known. If they are then Postgrey accepts the mail.

If they are not, the mail is temporarily rejected, that is grey-listed (hence the name Postgrey) as opposed to black-listed (always blocked) or white-listed (always allowed). Legitimate mail servers will retry after five minutes and the mail will then be accepted. After receiving 5 messages in a 35-day period the mails become trusted (added to the white-list) and are no longer delayed.

The idea is that spammers can’t be bothered to waste time trying to resend messages so they give up, knowing that people running Postgrey will also be running other spam filters, so their messages will probably be trapped anyway.

The advantages for people running Postgrey are

1. Spam is rejected before it is fully received, saving network capacity and usage allowances
2. The overhead on the spam filter is massively reduced as less is accepted and filtered
3. Users are less likely to get spam as the server has another level of protection

I thought that I had been running Postgrey for a few months, but I have just realised that I had not set it up properly and it was being bypassed. I have now rectified this and can see that it is working properly.

The only down-side is that mail will be delayed for at least 5 minutes from senders that are not yet on the white-list, but I can live with that if you can.