The Stasi Mediathek website published by the BStU is an amazing resource. One can search for example “Abteilung M” (English: Department M) and find a range of digitized original documents related to the Stasi’s postal surveillance.
This is an example of a letter that the Stasi Department M removed from the Deutsche Post in 1962. In the letter an East German student writes to her friend in the west about her dissatisfaction with the militarism and compulsory conscription in the east and says she is considering fleeing the GDR. Surely the Stasi placed her under a watchful eye due to the information they found through their postal surveillance. It’s also interesting the Stasi one, removed the letter altogether from the postal stream, likely in order to prevent negative sentiments from appearing in western press, and two, kept the document, perhaps as evidence against her at a later date, or potentially also as a result of the thoroughness of the bureaucratic surveillance system they had developed. An English translation can be found at the above link as well. (BStU)