The UvA holds a large collection of letters. Descriptions can be found in UvA CataloguePlus. Select Advanced search and then select Material Type: Manuscripts. Click in the drop-down menu on Any Field, choose Author if you want to search by sender or recipient, or choose Any Field if you want to search by location. Specify a start and end year if necessary, for example Start date 1850, End date 1870.
Part of the letters has been included in collection and archive inventories. Type the search terms: brieven OR correspondentie and choose source: Collection descriptions and inventories, from the drop-down menu.
Some letters are not yet listed in the online catalogue. Therefore, also consult the card catalogue in the Allard Pierson Reading Room.
The Catalogus Epistolarum Neerlandicarum lists descriptions of letters from the collections of cooperating Dutch heritage institutions, including the UvA, the Koninklijke Bibliotheek and the Literary Museum. Choose what you want to search in the Sender drop-down menu.
You can find published correspondences in UvA CataloguePlus: combine the name of the author and the search term brie, corresp? or letter? and choose UvA books+.