The International Institute of Social History in Amsterdam has created a database of the Yiddish letters in its archival holdings:

"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. [...] 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. Read more about the history of IISH.

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. A short abstract of every letter is provided."

The database can be downloaded on this page.

See also the full entry on the International Institute of Social History and its Yiddish holdings.