Question: How do I make sure my mail server isn’t an open relay?
Answer: First we should be clear on what an open relay is. According to spamhelp.org, an open relay is:
“a mail server that does not verify that it is authorised to send mail from the email address that a user is trying to send from. Therefore, users would be able to send email originating from any third-party email address they want.”
For example, someone could use yourmailserver.com to send emails from a fake account called firstname.lastname@example.org. It is a way to send spam while making sure their email address is not blocked. In some cases, they might even have software that creates fake email addresses that match the receiving server’s.
SpamHelp.org has an open relay test that you can use to check for open relays. If you find out your server does have an open relay, you should close it immediately. By default Postfix does not allow open rely, so you might want to check the security on your server if you did not open it. If you are running Postfix, you can fix an open relay by following the directions at The Drawingboard.