Another way to do this could be a Temp Trusted Seders list. If a user sends a email to firstname.lastname@example.org
it gets put on the temp list. After some interval (2 weeks?) it get deleted from the temp list. This would keep immediate responses out of junk folder and keep the trusted senders list or contacts (if SM goes this route) from getting a lot of clutter.
Also you could go a little farther and if 5 emails get sent to a recipient it gets put in the permeant list.
Junk Email filtered ISP