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/
All internal, incoming, and outgoing materials, regardless of whether or not they currently pass through a mail center; this includes envelopes, publications, postal cards, bulk mail, expedited mail, and individual packages up to 70 pounds that contain paper or publications.
It is also materials carried by the United States Postal Service (USPS), and all other carriers authorized by the USPS.