Lucky for you and I, Postfix is really good at using "as it sounds" configuration options. So, a client restriction would place a restriction on a mail client (for example, thunderbird) where a recipient is someone (or some service) on your actual machine.
Postfix is by far the easiest to work with, tweak, and understand mail server I've come across (for me, it may suck for everyone else
). So good luck in your venture. postfix.org actually has quite a few links and readme's on it's site.