>mbox should die;

Mbox is useful for backups and for migration between different email systems (that use different databases internally). Mbox is also fine if you only have a hundred or so email folders and only process a few dozen emails a day, say for personal use (e.g. Thunderbird or k9).

I agree that mbox is not okay for large scale mail servers. Maildir+ works much better in such cases.

So does Maildir; reusing it between Mutt and GNUs or Claws Mail should be a child's play.

Once you have tar to preserve perms just in case, your are done.

Mbox on big mailboxes it's hell, anyone can understand that linear parsing will be slow as hell. It's like looking up a word file in a dictionary word by word from A to Z instead of directly heading to the first word letter...