Last week we covered a problem where using an AppleWorks spreadsheet at the same time as Eudora, and then closing AppleWorks, could result in Eudora not being able to check for new mail.
MacFixIt reader Kiran Wagle claims to have identified the specific issue causing this conflict. Apparently, both AppleWorks and Eudora use the TemporaryItems folder located at /private/tmp/[user ID]/TemporaryItems. The problem occurs because of the different approach each product takes to this folder. When you close AppleWorks, it actually deletes the folder. That would probably be considered a "bad thing" by itself, but Eudora's approach ensures that a problem occurs. Instead of looking for the aforementioned folder by its path, Eudora -- through CarbonLib -- apparently stores the folder's directory ID. When the folder is removed, CarbonLib doesn't realize that it is gone, and tries to use the previous directory ID, which, of course, no longer exists.
The reason quitting Eudora and then relaunching it fixes the problem is that relaunching Eudora causes CarbonLib to find the folder over again (or create it if it's missing), and then store its new directory ID.