I have a similar problem.
Overhere in Holland we have on the indoor side of the mailopening a kind of brush system, consisting of two brushes: one on the top and one in front of the lower half of the opening.
For examples look over here:
https://www.google.nl/search?q=briev...A&ved=0CCIQsAQ
I even have added an extra flap on the inside (similar to the RONA mentioned before) and still a lot of air leaks in (despite of balanced ventilation with heat recovery).
I've made an infrared picture from the inside of the house:
De brievenbus - geen energierekening meer: energieneutraal woonhuis fam. Kriegsman Ootmarsum
So, I'm still curious for possible improvements...