What service to use to discuss a text document with the integration of work via email?

Question

Document flow

Good afternoon!
I am looking for a service for joint discussion of text documents. Text documents - plain plain text without any design. The main required feature is the ability to conveniently work with discussion threads without switching between windows in the interface.
Initial data:
1. There is a long email text - a description of some task. UserA sends this text to UserB's email.
2. UserB responds by text and sends the response.
3. Emails must go through some service so that the service understands the answers "by text" (breaking down the citation of the original email into parts). And automatically create an open discussion a la, as it is implemented on habrahabr.ru.
4. UserA responds to UserB's comments on the text of the letter and sends a response. The service complements the discussion.
At any time, it should be possible to open a discussion in full on the site. Interested in the most simple service that performs only this function. "Combines" are not interested.
For example, I see Basecamp as an ideal solution for working with discussions, but it does not have a bird's-eye view function where you can see the entire discussion with a branched structure, since they focus all screens exclusively on one task. I understand why this is done and fully support this idea, but sometimes you need a service to quickly discuss the problem as a whole.
Thanks in advance for ideas!