You could also bounce all email that you receive which isn't encrypted with your public key. If you think requiring all your contacts to set up GPG is too hard - what makes you think they'll adopt bitmessage?

(I know bitmessage has some different security properties than gpg+email -- but I think we'll all agree that if everyone and everything switched to email+gpg we'd be in a much better place from a security/privacy standpoint than we are now).