[SOLUTION]Mail does not quit or show old messages after 10.4-10.5 migration
1. Navigate to your Library folder inside your home folder.
2. Locate the Mail folder and drag it to your desktop.
3. Launch the Mail application.
4. Mail will now ask you if you want import your messages. DO NOT HIT CONTINUE YET.
5. Navigate back to your Library folder inside your home folder. Notice there is a NEW mail folder that was created. You cannot delete this folder yet because it is in use by mail so we will just rename this folder Mail2.
6. Drag your OLD mail folder located on your desktop to your Library folder located in your home folder.
7. Go back to the Mail application and click continue. Your messages and settings should import fine now.
8. Verify that your messages and settings imported correctly and quit Mail. Navigate back to your home Library folder located in your home folder and delete the Mail2 folder.
9. Launch the Mail application again and make sure everything functions correctly.
I found a fix to this by just messing around with the issue for awhile. I have found NO good resolutions online for this bug. Good luck to you all.
Macbook Pro, Mac OS X (10.5.6)