Drupal 7 e-mail driven forum (mailcomment)
This is in no way trivial since you will need to setup an email SERVER to react to certain emails and trigger a drupal 7 process. I have been tinkering with a bunch of modules and the most promising is og_mailinglist Within the module you will find a lot of information and Documentation of how to achieve this. You might even want to read through this to meet the makers

Even so the requirements are really tough and it is a project of several weeks if not months. most customers think this should be easy, well it is not. And it has security implications that have to be thoroughly solved. You are allowing an email to add content!

Good luck.