I have set up an SMTP server for personal use, and I used Postfix for that. However, Postfix has a lot of different options for how to configure it, and I'm guessing by configuring it incorrectly you can open it up to a lot of security problems, like unauthorized people sending spam mail. I followed the Ubuntu official guide mostly, but I had to step outside a little of them to get it work the way I want it.
So I want to make sure that my changes did not open up the SMTP server to some nasty security problem. What I'm looking for is a way to verify that, maybe some program designed to test that.
How do I know that my SMTP is secure?