Official mail

Official mail is mail sent from, or by, an authorized department of government, governmental agency or international organization and normally has some indication that it is official; a certifying cachet, return address or other means of identity, indicating its user. In some countries, postage stamps have been issued specifically for official mail.

Official stamps were intended for the same purpose as regular postage stamps except that they were unavailable for purchase at post offices. While most stamps are denominated some have no value imprint.

Some of the earliest official mail were the messages sent by kings, emperors and other rulers. The Cursus publicus was the official mail courier (and transportation) system created by the Roman Emperor, Augustus. Much later, one of the functions of the Thurn-und-Taxis-Post postal system was to carry the mail of Emperor Maximilan I in the Holy Roman Empire.

Official mail stamps are related to the end of franking privilege that granted certain elected officials of a government the privilege to send mail for free. The person usually signed their name on the mail but it was a highly abused system. In the UK the rules were changed requiring the sender to indicate the date and place of mailing in addition to their signature, but this too was abused and this led, in part, to the introduction of postage stamps.

Most governments use envelopes with an indicia indicating its official status; the United Kingdom's official mail most often shows, O.H.M.S. or On His/Her Majesty's Service, the United States uses the silhouette of an eagle, Ethiopia shows a hand holding a cleft stick with a letter is wedged into it, while Ireland uses a harp.

After the civil war, the United States Post Office Department had a large deficit and according to the Postmaster General's 1869 report 31,933 had the franking privilege at an estimated cost of $5 million, so in 1872 the Republican party included a proposal to eliminate the free frank as part of their election platform. Between 1873 and 1891 there was no free franking in the USA because special official mail stamps had been issued, but by 1891 free franking had been restored.

...
Wikipedia