I really dislike systems that conflate e-mail with forums/bboards. It's one of the reasons I stopped researching GNUCash (a free competitor to QuickBooks), though they go to the archaic extreme of using mailing lists instead of a real bboard. While I'm sure there are cases where it works well, personally I think it's a bad idea to create a system where we can't assume that everyone in the discussion has easy access to all previous posts (even good spam filters sometimes produce false positives, not to mention inadvertent deletions. Or to put it in a way other people here may appreciate, there is no yellow box in the e-mail client.
Technically, I wonder how one would authenticate post-by-email? I've never given the problem much thought, but I'd want something stronger than just checking the (easily spoofed) From: address.