I know most people can't use it due to needing email to be instant, but at least for my personal mail I've found greylisting to work wonders. What it does is t

I know most people can't use it due to needing email to be instant, but at least for my personal mail I've found greylisting to work wonders. What it does is tell the sender that it is busy/something is wrong and to call back later. It then makes a note of this. When the person calls back it will then accept the mail and add the address to a whitelist. This works because most spammers don't use a rfc complaint mail server and don't care to call back. It is cheaper to just move onto the next target.