From:
John Leach's Encyclopedia of Postal Authorities :

I

Icaria - International Organisations
Ionian Islands - Ivory Coast

Icaria - International Organisations

Icaria
ICC
Iceland
Idar
IEF
Ifni
Ikaria
Ile Rouad
Ili Republic
Imperial Japanese Government
Inde
India
Indian Convention States
Indian Expeditionary Forces (IEF)
Indian Feudatory States
Indian Native States
Indian Overseas Forces
Indo-China
Indo-China Territories
Indo-China (Indian Forces)
Indo-Chinese Post Offices Abroad
Indonesia
Indore
Ingermanland
Inhambane
Inini
Inner Mongolia
International Commission in Indo-China (ICC)
International Court of Justice (The Hague)
International Education Office
International Labour Office
International Organisations
International Refugees Organisation
International Telecommunications Union

Ionian Islands - Ivory Coast

Ionian Islands
Ionian Islands (German Occupation)
Ionian Islands (Italian Occupation)
Iran
Iraq
Iraq (British Occupation)
Ireland
Irian Barat
Island
Isle of Man
Isole Italiane dell' Egeo
Isole Jonie
Israel
Istria
Istria (Yugoslav Occupation)
ITA-Karjala
Italian Austria
Italian Colonies (General Issues)
Italian Colonies (British Occupation)
Italian East Africa
Italian Levant
Italian Occupation Issues
Italian Post Offices Abroad
Italian Post Offices in the Turkish Empire
Italian Social Republic
Italian Somaliland
Italian States
Italy
Italy (Austrian Occupation)
Ithaca (Italian Occupation)
Ivory Coast
Ivory Coast (French Colony)