How to recover an erased mail box in OE6?

I use "message rules" to sort incoming mail into various mailboxes. One of this has disappeared - I think I accidentally erased it! Searching in Winows Explorer finds a list of *.dbx files which doesn't include the missing mailbox. Highlighting "Folders.dbx" and then clicking "open containg mailbox" shows a folder with the name of the missing mailbox but with a "bak" extension. How can I get this mailbox back?