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.).