Understanding Postfix relay config

HI all,

Could someone quickly run through the essentials of a ‘normal’ relay set-up on Postfix. My understanding is that, out of the box, it will only forward mail sent from nodes on its subnet, or ranges specified in the mynetworks directive. So, the big question is this: how do we accommodate mobile clients who might be sending from any IP anywhere? The reason I ask is that until I added the public IP of my router to the ‘mynetworks’ entry, I was getting relay denied. I feel as though Postfix should be able to recognise that the incoming send request is from one of the accounts it is hosting, no? What am I missing? I’ve asked this question over on the Virtualmin forum so apologies if it looks familiar :)

If I can set all this up in Virtualmin, that would be of interest too, though config files hold no fear for me ;-)

Merci et bon weekend Mark