While I agree with both of you that they are different mediums and indeed they are, the actual structure of a forum thread could be viewed as a simplified mail thread. Generalizing a sub-forum could be regarded as a single mailing list.
Regarding the content and the codes of the messages, I'm pretty sure that today's mail readers can actually take care of all the extra formatting added by editors. Either by rendering them through the use of plugins and such or by filtering them out!
The reason I propose something like this, is that although I want to read several forums around the internet, I found the presentation and organization of the information, chaotic. Probably, there are other people too who share my opinion.
Furthermore, the basic functionality of my idea is actually half-implemented, given that someone uses the parameter "Default Thread Subscription Mode" set to 'instant email notification'. Then for every new post in a thread you receive a new mail! Much like a real mailing list thread.
Given time/code and some simplifications, I think it's possible to have a forum to mailing list gateway as an option. I don't know if it's a sought after feature, but if it existed I would certainly use it.
Anyway. Thanks about the insight guys.