Information about mail can be found in FMR 102-192
Postal Information and Resources - http://pe.usps.gov/
United States Postal Services - http://www.usps.com/
United Parcel Service - http://www.ups.com/
Federal Express Corporation - http://www.fedex.com/
DHL - http://www.dhl.com/
Department of State’s Diplomatic Pouch and Mail Division - http://www.state.gov/
Health and Human Services Mail Program - http://www.psc.gov
Military Postal Service Agency - http://hqdainet.army.mil/mpsa/
Department of Navy Mail Program - https://www.navsup.navy.mil/
Section 2 of Public Law 94-575, the Federal Records Management Amendments of 1976 (44 U.S.C. 2901-2904), as amended, that requires the Administrator of General Services to provide guidance and assistance to Federal agencies on records management and defines the processing of mail by Federal agencies as a records management activity.
The Domestic and International Mail Manual is the regulation of the United States Postal that governs its mail services.