My experience of Gmail is good as well, their spam filter is very accurate and learns quickly. IMAP works a treat, any system that doesn't support it is not worth using unless you have no choice.
Buy
Firetrust Mailwasher Pro which will filter any spam (with the option to boumce as per a non-existant address for any unknown sender)
(free Lite-version available)
Bouncing back spam as if the address is non-existant sounds like a great idea but in my experience it seems to generate more spam rather than stop it. Anyway spammers simply don't care whether the addresses they send to are genuine or not. They won't waste any time on checking for bounced back emails and deleting those addresses from their lists.