Highly experimental, blah blah ...
On data command, this plugin is pre-triggered (ie invocation before normal data behavior.
Examines the sender, resolve to MX, then try to connect back.
Some spammers do not use valid email adresse as from.
Provides a base class to extend to provide SMTP authentication
Not fully RFC compliant at the momment
TODO: Add support for cancel authentication
TODO: Correctly reset states.
TODO: When authentified, if client issues a RSET, the state is cleared.