After the 9/11 attacks, the USPTO established a Post Office Box in Arlington, Virginia (P.O. Box 2327, Arlington, VA 22202) for use on an emergency basis. Effective immediately, the Office will no longer accept patent-related correspondence at that address.
Use this address instead (with appropriate mail stops):
Commissioner for Patents
P.O. Box 1450
Alexandria, VA 22313-1450
Link.