The problem is probably that the mailbox has lost the
.mbox suffix in the filesystem.
Can you rebuild this mailbox, or does the
Mailbox > Rebuild command appear greyed out when the mailbox is selected in Mail?
What type of mail account is this (POP, IMAP, .Mac)? If IMAP or .Mac, what are your
Preferences > Accounts > Mailbox Behaviors > Junk settings?
BTW, if you’re running Mac OS X 10.3 (Panther), as your profile indicates, it would have been more appropriate to discuss this in the
Mail & Address Book - Mac OS X 10.3 & earlier forum. If the discussion applies to both Mail 1.x and Mail 2.x, it doesn’t really matter, but it’s difficult to know that in advance.
Mailbox Behaviors, for example, is called differently (
Special Mailboxes) in Mail 1.x.