Lion Mail: Delete mailboxes
If you no longer need a mailbox or any of the messages it contains, you can easily delete it. Do not delete a mailbox in the Finder because the change may not appear in Mail; always use Mail to delete mailboxes.
If a message says you can’t delete a mailbox, you may not have permission to delete a mailbox from On My Mac, or there may be network or server-related reasons.
To see your permissions for a mailbox, first display the Library folder (hidden by default) in your home folder: in the Finder, choose Go > “Go to Folder,” type ~/Library, and then click Go. In the ~/Library/Mail/V2/Mailboxes folder, select the mailbox, and then choose File > Get Info. Click the triangle next to Sharing & Permissions to see information about the permissions for the mailbox, and then make any necessary changes.
If you have problems deleting an Internet Message Access Protocol (IMAP) mailbox on the server, contact your email service provider.
When you delete a mailbox, rules that were applied to the mailbox are disabled in Mail preferences.
When you delete a Smart Mailbox, the messages it contains remain in their original location.
When you delete a mailbox that contains other mailboxes, messages in the top-level mailbox are deleted and the mailbox icon in the sidebar turns white; the other mailboxes and their messages still exist. If you delete a white mailbox, all of the mailboxes and messages it contains are deleted.