The Italian Academy of Philately and Postal History in its Italian Postal and Philatelic Dictionary describes post horn as follows:

Brass wind instrument, used by postillions to signal the arrival of a mail coach: this would prompt the staff of the postal stations to prepare the change of horses; in other instances: on hearing the post horn the ferrymen went to the riverside; the cities and castles opened the doors or sent guards to collect the mail. The post horn used different musical motifs, each with a specific meaning. Since the 18th century the post horn has become a symbol of the Post“.

While Wikipedia focuses more on its technical characteristics:

The post horn (also known as a postillion cornet) is a wind instrument…it was mainly used by postal couriers and stagecoaches, to signal their arrival and departure, or by postillions, especially during the eighteenth and nineteenth centuries [but also earlier]. Usually the instrument has a circular shape or is rolled up with three turns of the tube, although sometimes it can also be straight… The instrument is used as a logo of the national postal service in many countries, such as Switzerland, Sweden and Germany.

Since the Renaissance the post horn is a prominent element of the coat of arms of the Tasso (Taxis), De la Tour et Tassis and Thurn und Taxis dynasties. The cradle of Taxis dynasty is Cornello, a medieval village of the Brembana Valley, near Bergamo, the Taxis were the creators of an international modern postal service as we know it is today.

Here below, some beautiful videos on the post horn music:

Phillip McCann plays a kornett with Nidaros Brass Band, Trondhjemsnatt 2016

The Post Horn fanfare

The Post Horn fanfare

Post Horn in other languages

AlbanesePas Bri
Araboآخر القرن
BosniacoPost Sirena
Bulgaroпост рог
Catalanobanya de correus
CecoPost Roh
CroatoPost Rog
DanesePost Horn
EsperantoAfiŝu kornon
EstonePosti Saatmine
FinlandesePost Sarvi
FranceseCor Postal
Grecoμετά κέρατο
InglesePost Horn
IrlandesePost Adharc
IslandeseEftir Horn
ItalianoCorno di Posta
LettonePost Raga
LituanoPašto Ragas
Macedoneпост рог
NorvegesePost Horn
PolaccoPóg Pocztowy
RumenoPost Corn
Russoпочтовый рожок
Serboпост рог
SlovaccoPost roh
SlovenoPost Rog
SpagnoloCuerno Post
SvedesePost Horn
TurcoPosta Borusu
Ucrainoпост ріг
UngheresePost szarv