User To User Message Plugin

I am searching and searching – there must be a plugin about…

Requirements:

User(s) communicate to other users via messages/post. Almost a ‘ask seller a question’ plugin – where the question(s) and answer(s) is/are ‘logged’ and cannot be edited.

Message trail should be available to admin and users. (Again – Not editable!) – That answers ‘why not use email?’

By admin select-able:

1. Specific “user 1 can ‘message’ user 2 and vice verse.”
2. “user 1 can ‘message’ user 2,3 and 32 (example), but not others.
3. Selected users can ‘reply’.
4. ‘one to all’ or ‘admin to all’ function is not required.

I have searched and could not find anything even near my requirements. It would be nice if someone could point me in the right direction, suggest a plugin, etc.

Thank you.