Put his email address on your "blocked" list. But this is only gonna work as long as he does not need to send you work-related emails, of course!
Tell your boss. Say you're co-worker is emailing you pictures of "p-u-s-s-y"!! Look embarrassed when you say this. Do not say anything else. If your boss wants to see the emails, say you have deleted them! He will confront your co-worker -- and could say anything from telling him off for sending unncessary mail and slowing down the network to threatening him with the sack for sending pornography!!