I put an open relay detection and reporting system into operation in November 2005. It tried to relay test mails through each of the senders' IP trapped in my greylisting every half hour. If a relaying attempt was accepted, the corresponding IP would be reported to the ORDB. Then the ORDB would check whether it was an open relay or not. The idea was simple, but the system was very effective. It detected more than 1200 new open relays monthly, as confirmed by ORDB, during the first six months of its operation.

Numerous mail systems referred to the ORDB to block mail from open relays, so this automatic detection system greatly helped shrink the window of opportunity in which junk mail could successfully be relayed. Spammers were forced to adopt other spamming tactics than abusing open relays.

The change of spamming techniques proved the effectiveness of the ORDB. Unfortunately, the very success of the ORDB has led to its own decline...

Click to read the article this is in response to.