'Handle an APOP command.
Perform APOP authentication and complete the authorization process with
the L{_cbMailbox} callback function on success or the L{_ebMailbox}
and L{_ebUnexpected} errback functions on failure.
@type user: L{bytes}
@param user: A username.
@type digest: L{bytes}
@param digest: An MD5 digest string.'

def do_APOP(self, user, digest):

    d = defer.maybeDeferred(self.authenticateUserAPOP, user, digest)
    d.addCallbacks(self._cbMailbox, self._ebMailbox, callbackArgs=(user,)).addErrback(self._ebUnexpected)
