According to this post, there are mailboxes in the main terminal outside of baggage claim. Note - this is outside of security and you will need to reclear if you are in transit.
http://www.flyertalk.com/forum/10697677-post1.html
I'll move this to the Mid-Atlantic Forum so other travellers familiar with IAD can asssit.
FlyinHawaiian, Co-Moderator
United Mileage Plus Forum