"The word "legitimate" requires definition, "Revstampman".

Personally, I consider membership in the UPU as a "legitimate" requirement,
although I collect a lot of illegitimate postage stamps including those of the SOM."

"I hope to get to the SOM HQ in Rome this summer to see if they have a mail box
or letter-drop."

"Most stamp collectors collect by country or region, or topically. There is another option. Beginning in the 20th century, many stamps have been issued for use by international organizations. The League of Nations was one of the first to have stamps issued for its use.
[b]The League of Nations [/b]was founded in 1920 as a result of the Treaty of Versailles, one of the treaties that ended World War I. There were 58 member nations at its apex in 1935.
The League of Nations had the loftiest of goals including disarmament, preventing war through collective security, and settling disputes between countries through negotiation and diplomacy. It failed miserably in its primary objective, with wars raging in Spain, Ethiopia and China in the 1930s, leading up to the general conflagration of World War II, which began on Sept. 1, 1939.
The League of Nations had its headquarters in Geneva, Switzerland.
Various Swiss stamps were overprinted for use by the League of Nations, and by its subsidiary, the International Labor Bureau.
The Scott Classic Specialized Catalogue of Stamps and Covers 1840-1940 and the Scott Standard Postage Stamp Catalogue list these as Swiss Official stamps.
With WWII raging, the League of Nations adjourned Dec. 14, 1939, and did not reconvene until April 1946, for the purpose of dissolving the organization.

The League of Nations was succeeded by [b]the United Nations.[/b] Initially the United Nations comprised the Allied nations fighting against the Axis powers in WWII.
The U.N. Conference on International Organization was held beginning April 25, 1945, in San Francisco, Calif. The charter was signed by the 50 original members on June 26 of that year. Today the U.N. has 192 member states. The headquarters was established in New York City. Stamps have been issued for use from the U.N. offices in New York City; Geneva, Switzerland; and Vienna Austria. All are listed following the U.S. listings in the Scott standard catalog, Vol. 1, and in the Scott Specialized Catalogue of United States Stamps and Covers.
U.N. mail is carried by the U.S., Swiss and Austrian postal systems.
U.N./New York stamps are denominated in U.S. currency.
U.N./Geneva stamps are denominated in Swiss francs.
Stamps issued for the U.N./Vienna were denominated in Austrian schillings until Austria converted to the euro in 2002. They have been denominated in euros since that time.
Stamps issued by Switzerland for use by U.N. subsidiaries are listed as Swiss Official stamps in the Scott standard catalog, Vol. 6.
These include stamps for the International Bureau of Labor, the International Bureau of Education, the World Health Organization, the International Organization for Refugees, the European Office of the U.N., the World Meteorological Organization, the International Bureau of the Universal Postal Union, the International Telecommunication Union and the World Intellectual Property Organization.

Stamps issued for[b] UNESCO [/b](the U.N. Educational, Scientific and Cultural Organization) are listed as French Official stamps in the Scott standard catalog.

[b]The Council of Europe,[/b] headquartered in Strasbourg, France, was founded in 1949 with the chief aim of European Integration. French stamps issued for mail from the council's headquarters are listed as French Official stamps.

In addition to these stamps issued for international organizations, there have also been stamps issued by [b]international organizations for national use.[/b]
In 1962-63, the U.N. exercised temporary executive authority over Netherlands New Guinea and overprinted the former colony's stamps "UNTEA" for use during that period.
The U.N. administers the Serbian province of Kosovo and has issued stamps for use there since 2000. Stamps issued in 2000 for the U.N. Transitional Authority in East Timor are listed under Timor in the Scott standard catalog, Vol. 6."""