Following up on my problems with IMAP in Tiger, I’ve finally been rewarded for my diligent bug reporting with a solution!
I knew all along that incompatible mail bundles had the potential to break Mail.app, so I made sure to remove any bundles as soon as I found Mail wasn’t working. The only bundle I had was GPGMail.
What I didn’t know, and this is the key issue, perhaps I should post to the Apple forums about this as well, is that InputManager bundles can also cause the same sorts of problems. In my case, I had followed Scott’s suggestion to check out a little program called iAlert. I used it for a few days, then decided I didn’t need it so I turned it off and deleted the application. Unfortunately it had installed several other files it was also necessary to remove, including the culprit, /Library/InputManagers/iAlertHelper.
At least my IMAP works now! This means I was finally able to upgrade my home machine to Tiger with confidence I wouldn’t have to spend my free time fixing Andrea’s email.