I upgraded to Leopard quite some time ago and do not recall having any problems. I bought OS X Leopard to upgrade my parents' computers this weekend (got a good deal and in both cases, their Mail did not work following the upgrade. I thought this was odd since I've never had problems upgrading OS X in the past. I also found out today at work that a friend of mine just picked up a new Mac Mini this weekend and migrated from his old iMac (running Tiger) and experienced a similar problem so perhaps there is a problem upgrading to more recent versions of Leopard. I looked in the forums but did not readily find an answer to this problem. I tried a few things, including putting aside the preferences file (plist) for mail without success. The symptoms: Mail comes up for the first time after the upgrade or migration from another computer and says the conversion of the mail database to the new Mail format is complete, but the message window does not open automatically and when you open it, it does not show any of your old messages. The Mail app also does not actually get mail when you ask it to, just kind of sits there. I let it go for quite a while but nothing different happened. "Quit" does not quit it, you have to force quit it. Here's what I did to get my parents' computers fixed up: 1. Make sure Mail is not running. Force Quit if you have to. 2. Go into you <home>/Library folder. 3. Rename the "Mail" folder to something else (I used "Mail.old") 4. Start Mail.app. This time Mail will get mail from your mail server(s) but still will not show any old mail messages. 5. If any of the new incoming mail is important to you, save (export) it out somewhere. 6. Quit mail. A normal quit should work now. 7. Go into your <home>/Library folder again. 8. Delete the new "Mail" folder (do not delete the one you renamed earlier). 9. Rename the folder you renamed in step 3 ("Mail.old") back to "Mail". 10. Re-start Mail.app. Your old messages should be in your inbox and you should be able to send/receive mail. That's it. That fixed it for 3 user accounts used in both of my parents' iMacs.