Authentication command - requires response processing.
'mechanism' specifies which authentication mechanism is to
be used - the valid values are those listed in the 'auth'
element of 'esmtp_features'.
'authobject' must be a callable object taking a single argument:
data = authobject(challenge)
It will be called to process the server's challenge response; the
challenge argument it is passed will be a bytes. It should return
an ASCII string that will be base64 encoded and sent to the server.
Keyword arguments:
- initial_response_ok: Allow sending the RFC 4954 initial-response
to the AUTH command, if the authentication methods supports it.