Symptom: Emailer unpacks messages at startup before any accounts have even been checked!
Real problem: Emailer goes through it's usual downloading, but it doesn't unpack all the messages which have come in. The remaining messages are then unpacked the next time emailer goes through its unpacking routine. Of course, it usually leaves some other messages behind, again.
Problem Cause: the silly .DS_Store file which Mac OS X puts into every folder which has been opened by the Finder. The .DS_Store file is supposed to store info such as the view, the size of the window, and the like, so that the Finder can open it just like it was when it was last closed. Unfortunately, Classic apps don't know about .DS_Store files, so that they can get confused by their presence.