With my group, we started to use slack for email, but the transient nature of the chat did exactly what you describe. We ended up finding Discourse, a forum sy

With my group, we started to use slack for email, but the transient nature of the chat did exactly what you describe. We ended up finding Discourse, a forum system, and have been using that. Now, slack is more about simulating in person discussion (i.e. we rarely use it), and Discourse is more like emails pre-sorted by category. I have to say, it's working pretty well.