'Authorisation
- only possible if server has supplied a timestamp in initial greeting.
Args:
user     - mailbox user;
password - mailbox password.
NB: mailbox is locked by server from here to \'quit()\''

def apop(self, user, password):

    secret = bytes(password, self.encoding)
    m = self.timestamp.match(self.welcome)
    if (not m):
        raise error_proto('-ERR    APOP    not    supported    by    server')
    import hashlib
    digest = (m.group(1) + secret)
    digest = hashlib.md5(digest).hexdigest()
    return self._shortcmd(('APOP    %s    %s' % (user, digest)))
