Work out which mailboxes are unused - Mail cleanup

Hi There,

We have a work mail folder located at ~/Library/Mail that is over 70GB!

Our ISP is saying that the mail on their server is at 49GB so we suspect we have email that is in the Mail folder that is not being used.

How do we:

• Work out which mail is the unused 20GB so we can delete it?
• Archive unused mail to a backup?
• Easily view and browse the backed up email?

We've seen this article: http://www.macobserver.com/tmo/article/os-x-archiving-messages-from-apple-mail

But have also seen an 'Archive' folder in the sidebar in Mail - if we simply drag Mail/Mail Folders into this Archive folder, will than remove it from the server?

Hope that makes sense.

Cheers

Ben