If you haven't heard about this yet, congratulations on being removed from society so well! The mt-send-entry.cgi script in Movable Type allows anyone to send email to anyone using your server, much like formmail.

There is a fix, of sorts available.. although it's not a particularly good one. Spammers can still spam using that, they're just restricted somewhat.. I would suggest everyone just remove the thing altogether, there's no true need for it, it's not part of default MT config and anyone who really really wants to allow people to email entries should just code a better way of doing it. Like with validation of origins and such..