Can anyone give me some pointers/feedback on the best way to do this?

I have this box (Yellow Dog 3.0) setup and it is relaying mail outside,
but I want to make sure that this is kept secure.

I've read through docs before on the main.cf file, and I think that I
have it configured right, but wanted to make sure that I'm not leaving
anything open from the outside.

Thanks!

Chris