It’s notoriously tricky to get e-mail out of Leopard’s Mail.app if you’re dealing with more than a handful of messages. IMAP can be flakey, and the whole thing can be an exercise in futility, especially when all you you want to be able to do is “File” > “Export…” and save your mailboxes in some sort of standard e-mail format.
Well, you can.
Choose a mailbox. Right/Control-click. Choose “Archive Mailbox…”. Do the deed. You’ll want to view the Activity window (Cmd+0) to see when it’s done.
What Mail.app saves is a folder with a .mbox extension. The .mbox extension doesn’t actually do anything except give the folder a mailbox icon, but is slightly confusing because one of the standard e-mail interchange formats is called “mbox”, and Mail.app normally stores messages in “.mbox” folders which don’t contain messages in mbox format.
Archiving changes things, though. Archived “.mbox” folders do store the messages in mbox format, conveniently enough in a file called “mbox” within the folder. This file can be used by pretty much everything, from Thunderbird, to mutt and elm.