From:
John Leach's Encyclopedia of Postal Authorities :

M

Macao - Mayotte
Mecklenburg - Mytilene

Macao - Mayotte

Macao
Macau
Macedonia
Macedonia (German Occupation)
Madagascar & Dependencies
Madagascar (British Consular Mail)
Madagascar (French Post Offices)
Madeira
Mafeking
Mafia Island (British Occupation)
Magyarorszag
Mahe
Mahra Sultanate of Qishn & Socotra
Majunga (French Post Office)
Malacca
Malagasy Republic
Malawi
Malaya
Malaya (British Military Administration)
Malaya (Japanese Occupation)
Malaya (Thai Occupation)
Malayan Federation
Malayan Postal Union
Malaysia
Maldive Islands
Mali Federation
Mali Republic
Malta
Malmedy
Man
Manama
Manchukuo
Manchuria
Mariana Islands
Marianas Espanolas
Marianen
Marienwerder
Maroc
Marruecos
Marschall-Inseln
Marshall Inseln
Marshall Islands
Marshall Islands (German Colony)
Martinique
Mauritania
Mauritania (French Colony)
Mauritius
Mayotte

Mecklenburg - Mytilene

Mecklenburg-Schwerin
Mecklenburg-Strelitz
Mecklenburg-Vorpommern (Russian Zone)
MEF
Melaka
Memel (French Administration)
Memel (Lithuanian Occupation)
Mengkiang (Japanese Occupation)
Mengtse
Mesopotamia
Metelin
Mexico
Micronesia
Middle Congo
Middle East Forces (MEF)
Midway Islands
Modena
Moheli
Moldavia
Moldo-Wallachia
Moldova
Moluccas
Monaco
Mongolia
Mong-Tseu (Indo-Chinese Post Office)
Mongtze
Mont-Athos
Montenegro
Montenegro (Austrian Occupation)
Montenegro (German Occupation)
Montenegro (Italian Occupation)
Montenegro (Yugoslav Regional Issues)
Montserrat
Morocco
Morocco Agencies
Morocco (British Post Offices)
Morocco (French Post Offices)
Morocco (German Post Offices)
Morocco (Spanish Post Offices)
Morvi
Mosul (Indian Forces)
Mount Athos (Russian Post Office)
Moyen Congo
Mozambique
Mozambique Company
Mozambique Territories
Muscat
Muscat & Oman
Mustique
Mutawakelite Kingdom
Myanmar
Mytilene (Russian Post Office)