Being a mail provider, most of the things we do are to protect users and the servers. The in/out per day are really to prevent spammers and abuse. Also, I don't like charging people for overages. It never sits well.
As for managing spammers, just require some sort of ID verification? Or set the price point high enough that spammers wouldn't be interested?
$1.50/month for personal or $16/month for business is insanely cheap, I pay a lot more than that to Google. If you're too inexpensive, spammers will be attracted to your pricing.
Real businesses would have no problem paying upwards of $100/month, to be honest.