Ideally, don't. Self-hosting email is complicated, easy to get wrong (and dangerously wrong, where people could use your server as an open relay and send spam).
That said, if you really want to, make sure you're not accepting email except for what's destined for you. There are a bunch of postfix best-practice guides out there that can be easily found with a Google search. I don't host my own email, so I can't vouch for any.