D> * block email destined or CC'ed to a list of predefined mail
D> domains?

DS> With SPAMCONTROL, I think.

One can do it with stock "qmail" and an smtproute for each relevant domain
pointing to a rubber wall.

D> * use a list of authorized users which can send and
D> receive email?

DS> Could probably be done with Bruce Guenter's qmail-qfilter patch.

Authorization to receive mail can be handled by the normal user database
mechanisms. Authorization to send mail (via SMTP Relay) can be handled using
the ${RELAYCLIENT} mechanism, or (if that is insufficient because the SMTP
Relay clients are to be outside of one's own networks) by the smtp-auth patch
(as long as it is configured correctly (-:).