Yiddish letters can be found in several archives which were acquired by the IISH in the course of time. Some of these are from archives bought or deposited in the IISH after the coming to power of the German National-Socialist Party in 1933. The Institute was founded in 1935 to safeguard important archives and libraries of the leftist parties in Germany from destruction by the Nazis. Among these archives are that of the Menshevik leader Raphael Abramovič and the Russian Socialist-Revolutionary Party. During the German occupation of the Netherlands which lasted from 1940 till 1945, the Institute was closed and its contents confiscated. In 1944 the books and archives were shipped to Germany, but escaped destruction.
After the war, the greater part of the collection was found near Bremen in the British zone of occupation and sent back to Amsterdam in 1946. Other parts were confiscated by the Russians and brought to Moscow, they were only recently returned. After World War II, many important anarchist archives that include Yiddish letters came from the U.S.A. These included the archive of the German anarchist Rudolf Rocker and the Russian-American anarchists Senya Fléchine, Alexander Berkman, and Boris Yelensky.
Thanks to Dr Rena Fuks-Mansfeld, more than 700 of these Yiddish letters are now accessible through a database. The files can be searched on author, sender, destination, and period of time.
More: http://socialhistory.org/en/collections/yiddish-letters