From Moth Software Mainz:
Mail Archive X protects one of your Mac-based small business' most important assets - your email. After you've set up archiving, future archives can be created in one click. And these are smart archives - taking only what you have not archived before. All major email clients are supported: Apple Mail, Eudora, Outlook, Entourage, Powermail, Postbox and Thunderbird. Of course, Gmail and Exchange are supported, too. Archive as much of your email as you want, or archive and speed up your email client by removing the mails from the mail client. Search archived email as easily as you search email today. Mail Archive X automatically checks and verifies as good whenever mails are added to an archive. Easily export your email to Evernote, pdf, Filemaker or other formats to suit your needs.