But then you'll be using the mail to commit a crime and the postmaster general will come after you....
Correct. There is a SYSTEM in place to prevent voter fraud for mail in voting. Yes the state of CA publishes when they release most of their ballots, and it's kinda ridiculous. You could go burglarize all those mailboxes.
So mail in voting is very susceptible to fraud because the barrier is low. But at least there's a barrier. Some states make you mail in a copy of your ID or something. I remember having to do that in 2004.
Which brings me back to the idea that instead of mail in voting, online voting could be better. Perhaps require two factor authentication or something as an ID check. The easy barrier to breach privacy with mail in voting shows that online voting isn't any worse.