From:
John Leach's Encyclopedia of Postal Authorities :

S

Saar - Saxony
Scarpanto - Slovenia
Smirne - Southern Zone, Morocco
Spain - Szeged

Saar - Saxony

Saar
Saar (French Administration)
Saar (French Zone)
Saargebiet
Saarland
Sabah
Sachsen
Sahara Espanol
St Christopher
St Christopher Nevis & Anguilla
St Helena
St Kitts
St Kitts-Nevis
St Kitts Nevis & Anguilla
St Lucia
St Paul
St Pierre et Miquelon
St Thomas & Prince Islands
St Vincent
Ste Marie de Madagascar
Salonicco
Salonika (British Field Office)
Salonika (Italian Post Office)
Salonika (Russian Post Office)
Salonique
Samoa
Samoa (Kingdom)
Samoa I Sisifo
Samoa (New Zealand Administration)
Samos
San Marino
Sandjak d'Alexandrette
Santander
Sao Tome e Principe
Sarajevo Government (Bosnia)
Sarawak
Sarawak (British Military Administration)
Sarawak (Japanese Occupation)
Sardinia
Sarkari
Sarre
Saseno (Italian Occupation)
Saudi Arabia
Saurashtra
Saxony
Saxony (Russian Zone)

Scarpanto - Slovenia

Scarpanto
Schleswig
Schleswig-Holstein
Schweizer-Renecke
Scinde
Scotland
Scott Expedition
Scutari (Italian Post Office)
SDD
Seiyun
Selangor
Semyonov Regime
Senegal
Senegal (French Colony)
Senegambia & Niger
Serbia
Serbia (Austrian Occupation)
Serbia (German Occupation)
Serbia (Yugoslav Regional Issues)
Serbian Occupation Issues
Serbia and Montenegro
Seychelles
Seychelles Outer Islands
Shackleton Expedition
Shahpura
Shanghai
Shanghai (US Postal Agency)
Sharjah
Shensi-Kansu-Ninghsia
Sherifian Post
Shihr & Mukalla
Shqip-
SHS
Siam
Siam (Thailand)
Siam (British Post Offices)
Siberia
Siberia (Czechoslovak Army)
Sicily
Sierra Leone
Sikkim
Silesia
Simi
Sind
Singapore
Sinkiang
Sirmoor
Slesvig
Slovakia
Slovakia (Autonomous State)
Slovenia
Slovenia (Provincial Issues)
Slovenia (Yugoslav Regional Issues)
Slovenia (German Occupation)
Slovenia (Italian Occupation)

Smirne - Southern Zone, Morocco

Smirne (Italian Post Office)
Smyrne (Russian Post Office)
SO 1920
Solomon Islands
Somalia
Somalia (British Administration)
Somalia (British Military Administration)
Somalia (British Occupation)
Somaliland Protectorate
Soruth (Saurashtra)
Soudan
South Africa
South African Republic
South African Territories
South Arabian Federation
South Australia
South Bulgaria
South China
South East Saxony (Russian Zone)
South Georgia
South Georgia (Falkland Islands Dependencies)
South Georgia & South Sandwich Islands
South Kasai
South Korea
South Korea (North Korean Occupation)
South Lithuania (Russian Occupation)
South Orkneys (Falkland Islands Dependencies)
South Russia
South Shetlands (Falkland Islands Dependencies)
South Vietnam
South West Africa
South West China
Southern Cameroons
Southern Nigeria
Southern Rhodesia
Southern Yemen
Southern Zone, Morocco

Spain - Szeged

Spain
Spanish Guinea
Spanish Marianas
Spanish Morocco
Spanish Philippines
Spanish Post Offices Abroad
Spanish Sahara
Spanish West Africa
Srba Hrvata Slovena
Sremsko Baranjska Oblast (Croatia)
Sri Lanka
Srpska
Srpska Krajina (Croatia)
Stadt Post Basel
Stampalia
Stellaland
Stockholm
Straits Settlements
Strasbourg
STT-VUJA/STT-VUJNA
Sudan
Sudetenland
Suez Canal Company
Suidafrika
Sumatra (Japanese Occupation)
Sungei Ujong
Suomi
Surinam/Suriname
Sverige
Swaziland
Swaziland (Provisional Government)
Sweden
Swiss Cantonal Administration
Swiss Cantonal Issues
Swiss PTT
Swiss Transitional Issues
Switzerland
Syme
Syria
Syria (French Occupation)
Szechwan
Szeged