def unsubscribe(mailbox)
      send_command("UNSUBSCRIBE", mailbox)
    end
Sends a UNSUBSCRIBE command to remove the specified +mailbox+ name
from the server's set of "active" or "subscribed" mailboxes.

A Net::IMAP::NoResponseError is raised if +mailbox+ cannot be
unsubscribed from, for instance because the client is not currently
subscribed to it.