I just started up Mail.app on 10.4. It did its import thing, importing mail from the older mail.app I guess, and then downloaded the new mail from the server. After it had finished downloading, I clicked on one of the unread mails, and the damn thing crashed. And everytime I start up Mail.app, it just crashes. This is not good.
Looking at the crash report, it seems it might be the gpg module I installed using fink. I'm going to try to recompile those modules. If that does not work than I'll probably be installing OS X Tiger from scratch. Dont want anything broken on the system.
The directory ~/Library/Mail/Bundles contains a directory called GPGMail.mailbundle which I moved from their, and the Mail.app came up properly.