The Allied Postal System was the postal system of the Union of Allied Planets. Used throughout The Verse, a Postmaster was the primary official who handled the "Post" or mail, They could be found on most Spacestations, Planets, and Moons. The APS apparently had the power to terminate a Postmaster (or at least revoke their franchise) instantly, although there seem to have been only a very limited number of potential causes for such, the one cited in "The Message" being "human transport on a postal route", punishable (according to Lieutenant Womack) by five to ten years.
APS traffic appears subject to at least some degree of Allied governmental and/or Blue Sun Corporation scrutiny, given the way Womack deals with Amnon.