honestly... not sure about the free services.. but hushmail if you use thier pay version.. you can block all emails not excrypted, it has an autoresponder now too i think, spam filters etc... i think it would be worth the cash if i had it...
guaranteed no emails unless encrypted... sounds good to me!!
(pgp or hush/cyber) ... seems pretty secure especially w/ 2048 bit encryption on thier servers... how much does that stuff cost these days anyway?