Spammers are clever. They have figured out how to use a feature of MovableType to send spam from your server, looking like it came from you! Imagine the glee of MT users.
Fortunately, there are solutions, but they require a positive action, which can be found below: movabletype.org : Support Forum. There’s also a new version of the offending file, with better security.
My chosen answer, the easiest, is just to delete the offending file that allows this to work. Your mileage may vary, but do something to keep from being used by spammers.