I know Hotmail has a 'safe list' feature, preventing any messages from certain addresses (that are not on your contact list) from going to the junk mail folder. Yahoo might have it too.

Julie

Mulder: Imagine if you could come back and take out five people who had caused you to suffer. Who would they be?
Scully: I only get five?
Mulder: I remembered your birthday this year, didn't I, Scully?

(The X-Files)