http://spamlinks.net/prevent-secure-relay-test.htm

When configuring a mail server, you always want to do a little testing to make sure you don't have an open relay that can be used by spammers.

If your mail server is an open relay, it will cause you much grief and may not be easy to recover from.

http://www.mxtoolbox.com/diagnostic.aspx

Run these commands on the mail server, Windows or Linux:

telnet relay-test.mail-abuse.org

telnet rt.njabl.org 2500