Found an agent called "EmailSiphon" in my stats which got me twitchin. At http://www-stu.cai.cam.ac.uk/news/n01.html
I found the following:
We've taken care of it on this server by a little Rewrite rule added to the server which simply checks the UserAgent and returns an error if it is indeed EmailSiphon.
Can something like this be implemented here? What about other email spiders?