You should be able to find the E-mail in the maildir directory... If this is an E-mail address for a full user, just log in with that user to shell or FTP, and you should be able to track it down. If it's a mailbox with out a user, I suspect you can access the folder somewhere, but I don't know where.
Is your E-mail box or user at it's storage limit? If that's the case it might be simple to just up the storage so you can work with the E-mail from T-bird of webmail.