some users prefer forums and some mailing lists. Both groups are too big to ignore and both have good reasons for their decision, so one should not simply close any of the two platforms.
OK. I thought that might be the case, but good to confirm.
What's missing is a technical solution where you can interact with a forum via a mailing list so that every user can use what he likes best. But we have not found a really suitable software application. We're interested in any practicable suggestion to solve this scattering.
Use a private NNTP server to store the conversation articles, and one of the WWW-based news reader/post tools to provide the forum version; use mail2news to link the mailing list to the NNTP environment. Easy to manage, known to work at scale, uses standard components (no development required, just configuration), does not mandate special software at the client side. Shouldn't take you more than a day or two to set up.