OS X V10.7 Lion :: Mailboxes Vanished From Sidebar

I have Mail on my wife's MacBook Pro (running Lion 10.7.4) set up to access our host's mail server via IMAP. The inbox appears in the sidebar as 'Guided Tours' under 'Inbox'. She's set up a whole bunch of custom mailboxes which normally appear in her sidebar under the heading 'Guided Tours'. After shutting the Mac down for a week while she was away the only mailboxes there are Sent and Trash. The mail, in all its folders and subfolders, is all there on the server - I can see it via our host's horrible webmail interface. I've looked at Mail's preferences and come up with nothing - 'Mailbox behaviours' etc. are no help. The mailboxes had also disappeared from inside [URL] so I restored them from the last Time Machine backup from before she went away, but Mail wouldnt see them after quitting and relaunching or even after restarting the Mac. I then tried restoring the entire ~/Library/Mail folder from TM, but still no good. Finally, I deleted the whole account and recreated it from scratch, and this is where it gets even more interesting... This appeared to work - hurrah! Back in business with all the mailboxes showing as they should, and apparently fully functional... until I looked at her Mac about an hour later - when it hadnt even been to sleep, let alone shut down - and they'd vanished again! This time though, the mailboxes are still there in the INBOX.mbox folder... except they're all empty! For what it's worth, as well as being there on the server, the mailboxes are, fortunately, all accessible and usable from her iPad and iPhone, so she can use those for now.