This is about the application Mail in OSX Tiger, right?
If you want to delete a mailbox, you need to delete the mail accound.
- Open Mail
- Go to Preferences (from the "mail" menu on top off the screen)
- Go to the accounts selection (second tab)
- Select the account you want to delete
- Click on the "-" at the bottom off the screen
- Now the account will be deleted (and also to mailbox)
Hopefully this solved your problem.
(see button above this message)