EDITED
The excellent Privatemsg module provides a wealth of functionality but some sites might not need/want this degree of complexity. Rather than starting with something complex and disabling/removing features, I would like to think about ways to build something relatively simple from basics.
Using the basic infrastructure of D7 what are the ways one could create a simple messaging system that allows user A to message user B, and both of these corresponding users to delete this message individually (i.e. user A deletes message 1, but user B does not; therefore user B can still see message 1)?
How might message threads (i.e. successive related messages) be managed and displayed?
How well would the solution scale (e.g. 1,000,000 messages, 100,000 threads, etc.).
Thank you.