Google Outbound Services Configuration Guide - Manual (Page 99)

106 Outbound Services Configuration Guide
5. Confirm that your mail server is not an open relay. An open relay will make
your mail server vulnerable to hijacking from spammers and will most likely
cause an interruption in service.
Use an external open relay test, such as http://www.mxtoolbox.com/
diagnostic.aspx or http://www.spamhelp.org/shopenrelay/ . If the result
shows that you have an open relay, correct your private relay settings. If you
see an uncertain response (such as “maybe” or “warning”) then check that
your private relay settings are correct.
See “Set Up Reinjection” on page 104 for the correct private relay settings.