Great little app. Fills a void in Apple Mail. If you want to archive old email and still be able to find or search the archives without launching Mail, then this little app is for you. Unlike the other available solution; Mail Scripts, Mail Saver will rename the email in an archive folder to make searching quick without even using Spotlight. Since Leopard, we can now view emlx files in the Finder with QuickLook and Search them with Spotlight. Mail Saver adds the needed functionality of renaming the archived emails in human-readable form. And because the individual emails remain in emlx format, they can be opened or imported back into Mail.

Apple Mail's "Archive" function creates an mbox file that is not human-readable in the Finder, forcing the user to re-import the mbox in order to read the emails. This function is intended as an export feature for other apps to import. Not to create a human-readable archive.

The Mail Scripts solution is Applescript based and thus very, very, slow. Although it does create a mail folder with Finder readable emlx files, they are numbered files, with meaningless filenames.

You can do this for free in the new version of Mail. Select your mailbox and export it from the menus.