U.S. Citizenship and Immigration Services (USCIS) announced today a new address format for offices within the National Capital Region (NCR), affecting all USCIS headquarters offices, the Arlington Asylum Office, and the Washington District Office. In order to ensure the timely delivery of USCIS mail, correspondence addressed to the affected offices should list a unique mailstop and corresponding ZIP code + 4 number.
For example, the new format for the Information and Customer Service Division, which handles all general inquiries, is:

Information and Customer Service Division MS 2260
U.S. Citizenship & Immigration Services
111 Massachusetts Ave. N.W.
Washington, D.C. 20529-2260

See also: USCIS Zipcodes and Plus Four Chart