Thank for all the great info.
The current site is closed, so members knowing about each other is simply through internal discussion boards.
The purpose of the "sticky mail" is to allow sending additional information to others, such as a private e-mail address, or to have a private conversation. (e-mails are private even inside.)
How it would hook in? The current system is (CO Mail) allows regular send, receive, forward, reply, delete. Users can pick a single recipient from the member pick list. The whole system is a single PHP file which calls itself for various functions and as the POST result, with a MySQL DB back-end.
Variables are passed to it through session vars, and security is simply an include prior to the header which checks if the user is logged in - if not they are redirected to the login page. So - a system can be bolted to any site if there is no security feature exept for passing a list of possible recipients. This can be in an array if small or a table, which is my case.
I think the simplicy of the system would allow it to be bolted to most systems.
I just don't know where to go from here, nor do I want to re-invent the wheel...