The financially strapped U.S. Postal Service is running into opposition from historic preservationists as the agency tries to cut losses by selling off buildings.

The postal service lost $15.9 billion last year, after losing $5.1 billion in 2011 -- as online services continue to replace money-making mail deliveries.

Hundreds of post offices are on the National Register of Historic Places, which largely protects them from being demolished, or are protected under deals with new owners....