What is an open relay?
An open relay, also known as third-party relay, is an email server that allows anyone on the Internet to connect to it and send email. For example, 'Joe User', dialled in through 'Big ISP' can connect to the open relay and send his email to any destination he chooses. This abuse is popular with spammers.
Checking for open relay
A way to test your server is to telnet to the IP address in question on port 25 from a different Internet Service Provider and manually initiate an SMTP transaction. If you can send mail from yourself from the different ISP, your server is an open relay.
Why should I close my open relay?
Your server may encounter severe problems if it is used to send unsolicited bulk email (spam). These problems may include the server crashing or being blocked from sending email to some domains.
Getting out of AOL's block list
All of the servers in our database are tested once every 24 hours by an automated process. If your open relay is closed within 24 hours of the last test, it will be marked 'closed' following the next test. Other processes will then take this new information, pull your IP from our block file, and push the changes into production. These 'push' and 'pull' production changes occur several times a day.