Granted, the mailbox was around 1 GB - but the migration process was the most painful, most annoying, and idiosyncratic thing since Windows ME. Beware - the performance was quite poor on the thing from what I saw, certainly compared to what I’m used to. Once the user opened up his mailbox, it promptly began scanning all his folders. He ended up killing the process because he thought it had hung. That ended up corrupting the whole mailbox store. Now it won’t start up at all.

Bottom line: Thunderbird has matured - the UI is much nicer, and there is a lot of polish about it. but there is still some dirt under the hood that I detected. Performance isn’t great, and the Backup/Restore process is unreliable and loses bits and pieces of mail - something I don’t understand.