From:
John Leach's Encyclopedia of Postal Authorities :

H

Hadhramaut - Hyderabad

Hadhramaut - Hyderabad

Hadhramaut
Haiti
Hamburg
Hanover
Hatay
Haute Volta
Hawaii
Heilungkiang
Hejaz
Hejaz-Nejd
Heligoland
Hellas
Helvetia
Herm
Herzegovina
Hoi-Hao (Indo-Chinese Post Office)
Holkar
Holland
Holstein
Honduras
Hong Kong
Hong Kong (Japanese Occupation)
Horta
Horthy Regime
Hrvatska
Hungary
Hungary (French Occupation)
Hungary (Rumanian Occupation)
Hungary (Serbian Occupation)
Hyderabad