The USPS has revised the Employee and Labor Relations Manual (ELM) to clarify that the ban on smoking includes electronic cigarettes:
ELM 882.1 says
Smoking is strictly prohibited in all buildings or office space (including service lobbies) owned or leased by the U.S. Postal Service. There will be no indoor smoking permitted by any occupant of such space. Local managers, with input from employee representatives, may decide whether or not to permit smoking in designated outdoor locations on Postal Service property.