There are similar challenges as you expand the mailing list gateway concept.
How do you keep spammers or trolls from forging the addresses of real members? And, of course, there is no way to cancel an e-mail once it has been sent out to your members... I hope there's a viable strategy with some encryption tricks and custom headers that will help. Lots to think about!
Still, what's the answer to any of these questions? Work through the logic of the problem, pick out whatever solutions might exist, if any, and present them to the user of the mod as options in Admin-CP! No problem!
Have you found sources for the protocols for Usenet? I think we're each going to need to study the parts of this application we're personally interested in and share our research to help with the coding.
Be well,
Mike D.