[Dovecot] Mailbox isn't a valid mbox file

Keep getting this error within Squirrelmail (and other clients) on one particular user's mailbox file:

ERROR: Could not complete request. Query: SELECT "INBOX.Backup" Reason Given: Mailbox isn't a valid mbox file

I've snipped the first and last message off of it, still no love.

mailserver# cat FILE | od -c | less 0000000 F r o m p r o m o t i o n s @ 0000020 e m e s s a g e . c i s c o . c

So apparently there isn't a control character in the first message that'd be causing this either.

Anyone have any ideas? Only one user out of many is having this issue, but he's whiny...

-- Jay Chandler Network Administrator Chapman University