I have no experience with this. Could you please suggest a concrete set of applications for this solution or a how-to? I will then try to look at this.
INN is the most common NNTP server. http://www.eyrie.org/~eagle/faqs/inn.html for more information about INN. Mail2news is part of the INN package. Instructions for setting up mail2news are in the INN package. One WWW-based NNTP client is http://web-news.sourceforge.net/. There are lots more; I pulled this one at random. Basically, what you want to do is use the software originally designed for Usenet news, and create a two-way gateway from a newsgroup to a mailing list. The users that like "pull" content (forum users) interact with the newsgroup via a NNTP client, the rest of us use the mailing list via email. Both sets of users see the same content. Note that the use of the "Usenet news software" does NOT imply implementing a full news server. You don't have to feed or accept feeds from other people; in fact, you probably want to lock down the NNTP server to the IP addresses of the WWW-based client machine to avoid Google, etc from skimming out your content.