e-mail/bboard integration was done for ACES, which you can download from aD. Unfortunately, rather than working with the existing bboard module they implemented a brand-new one based on general comments which is lacking in other ways. ACS 4 brings another total redo of bboard to the table. I bet it gets rewritten again for ACS 5.
Without getting too deeply into philisophical contemplation ("just how many times should one rewrite something as fundamentally simple as a bboard module?") the pieces are all there to integrate e-mail with ACS3 bboards and later ACS4 bboards. The hack done for ACES uses Oracle routines to grab the e-mail messages but of course non-Oracle pieces exist for web mail, which is included in OpenACS 3.2!
I'd love it because I'm going to need it myself later this spring or early summer.
Pieces pieces everywhere ...
I'd love to see someone do an integration with OpenACS 3.2. The "mail to OpenACS which then alerts members who've asked for them" paradigm means there are no mail loops unless someone registers with the system 's e-mail address as their user name. It would be easy enough to refuse such registrations...