An open mail relay or simply an open relay is a mail server that allows non local users to send mail to another non local users. I mean, the mail server processes mail messages where neither the sender nor the recipient are users of the local mail system. This is the perfect scenario for spammers to abuse your system.
There are mail servers such as Postfix that are not open relays by default. To check if your mail server is an open relay take a look at the following links:
- Network Abuse Clearinghouse is intended to help the Internet community to report and control network abuse and abusive users. It provides a mail relay testing service.
- The Open Relay DataBase stores IP-addresses of verified open SMTP relays and provides useful information on fighting spammers.
- Another testing tool here
- There are tons of sites on this subject. Try google or freshmeat for example.
I found this link very useful.
Be carefull! I’ve been told that testing at ORDB is not a good thing if you are open relay cause they add tested systems to its blacklist. Try to do tests at another place that does not automatically add u to a blacklist if you are open.