'SMTP service extensions
@return: the SMTP service extensions that are supported.
@rtype: L{dict} with L{bytes} keys and a value of either L{None} or a
L{list} of L{bytes}.'

def extensions(self):

    ext = {'AUTH': _keys(self.challengers)}
    if (self.canStartTLS and (not self.startedTLS)):
        ext['STARTTLS'] = None
    return ext
