after installing mountain lion my mail program stops on "upgrading mail database"

I installed Mountain Lion on my 27" imac (late 2009) and had no problems until I got to the Mail program. When launching Mail a utility attempts to "upgrade" the previous database of messages. When I started the upgrade process, the program almost immediately crashes and dumps me out. I have a considerable amount of messages in my inbox and I'm sure this probably has something to do with the problem. While installing Mountain Lion on my macbook pro I experienced the exact same problem only it does not dump out of the window. It just runs indefinitely without ever completing the task. The progress bar just sits on about 1/3 complete. After doing the first M/L installation on the imac I attempted to pare down my Inbox by archiving some of the messages but it did not prevent the same problem from happening on the macbook.