From:
John Leach's Encyclopedia of Postal Authorities :

XYZ

Yanoan - Yunnansen
Zadar - Zurich

Yanoan - Yunnansen

Yanoan
Yemen
Yemen Arab Republic
Yemen (Mutawakelite Kingdom)
Yemen PDR
Yemen Republic
Yokohama
Yugoslavia
Yugoslavia (Kingdom)
Yugoslavia (Democratic Federation)
Yugoslav Government in Exile
Yugoslav Military Government in Trieste
Yugoslav Military Government in Venezia Giulia
Yugoslav Occupation Issues
Yunnan
Yunnanfu (Indo-Chinese Post Office)
Yunnansen

Zadar - Zurich

Zadar
Z. Afr. Republiek
Zaire
Zákinthos
Zambezia
Zambia
Zante (German Occupation)
Zanzibar
Zanzibar (French Post Office)
Zanzibar (German Postal Agency)
ZAR
Zara
Zil Eloigne Sesel
Zil Elwagne Sesel
Zil Elwannyen Sesel
Zimbabwe
Zuid Afrikaansche Republiek
Zuidwest Afrika
Zululand
Zurich