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

def apop(self, user, secret):

    m = self.timestamp.match(self.welcome)
    if (not m):
        raise error_proto('-ERR    APOP    not    supported    by    server')
    import hashlib
    digest = hashlib.md5((m.group(1) + secret)).digest()
    digest = ''.join(map((lambda x: ('%02x' % ord(x))), digest))
    return self._shortcmd(('APOP    %s    %s' % (user, digest)))
