Here is a list of various mail servers and how to secure them against open relay. Spammers are bad enough of a nuisance without contributing to the problem.
Postfix is not, by default, an open relay. More information (including explanations of some confusion behavior in early versions) can be found in the Postfix FAQ, http://www.postfix.org/SMTPD_ACCESS_README.html, and the UCE Controls, http://www.postfix.org/uce.html, section of the Postfix Configuration manual.
This list is dated I’m afraid.
[tags]Mail Relay, Server Configuration, Spammers[/tags]