This guide will teach you how to make sure that your SMTP mail server is not a relay server.
An open relay server allows people to use your mail server to deliver spam.
If you have an email server on Ubuntu or other Debian based operating system and you are setting it up using postfix and courier then by default they are not going to be open relay servers. There is a very simple way of testing. The mail-abuse.org offers an automated test suite. The way you run this is by logging into your server, this can be done through ssh.
From your mail server run:
telnet relay-test.mail-abuse.org
This will attempt to connect back to your machine and run a series of mail relaying tests against it.
Upon completion you should recieve the following message:
System appeared to reject relay attempts
If you are inadvertently running as a relay consult your mail server documentation for tips on how to prevent it.