Why does my IMAP client show that I have mailboxes named “#mhinbox”, “#mh”, “#shared”, “#ftp”, “#news”, and “#public”?
These are IMAP namespace names. They represent other hierarchies in which messages may exist. These hierarchies may not necessarily exist on a server, but the namespace name is still in the namespace list in order to mark it as reserved. A few poorly-designed clients display all namespace names as if they were top-level mailboxes in a user’s list of mailboxes, whether or not they actually exist. This is a flaw in those clients.
