I heard about it at the Stasi museum in Leipzig. Basically, the plan was to try and spark some sort of conflict in the West, relying on neo-Nazi ideals.
What confuses me is that, according to the museum (I THINK; it was in German and I can't read it well), the MfS added their own fake West German postmarks to the letters before they were distributed. Why was this done, instead of just... Having somebody in the West send the letters normally? How would they have been snuck into the mail stream? Is there anywhere I can get more information about this?
Hello there!
As Warsaw Pact disinformation campaigns form part of my research I will try to provide you an answer.
As you stated, the purpose of sending these letters was to fuel political tensions in the West. In America for example, the KGB even went as far as demolishing Jewish graves, or sending bomb threats to African American schools on behalf of Jews and writing fake KKK letters to African diplomats at the UN. It was partly to destabilize (although they did not assume it to be anything more than just creating a minor inconvenience), but also to produce propaganda for use at home. Soviet newspapers would for example report on these blatantly racist incidents to support their narrative that the capitalist-imperialist West is inherently racist.
The Czechoslovak secret service at some point in the 1950's made fake orders with West-German firms on behalf of the West-German government, with the purpose that confusion would arise and ultimately damage the reputation and reliability of the government in the eyes of its own business people. Even prank calls belonged to the Czech repertoire.
This practice of covert political manipulation had its origins in pre-war Soviet ideas about confusing and disinforming its enemies to compensate for lagging behind in military strength. This was mostly military-strategic disinformation. After 1945, deliberate political manipulation was perfected by Ivan Agayants, a brilliant KGB officer of Armenian descent who succesfully wielded these tactics as KGB resident in France. In the 1950's he became a teacher at the KGB school, and by 1959 KGB chief Shelepin made him responsible for setting up a new, specialist disinformation department within the KGB. The Soviet Union wanted to beat the West through political means, and not war. This required a coordinating unit for political warfare. In KGB jargon it was called "Active measures".
All Warsaw Pact countries followed the KGB's example, and within the MfS it was Section X of the Main Intelligence Department. It was headed for most of its existence by Rolf Wagenbreth.
These units typically consisted of several dozen officers, some of whom claimed their work was a lot like that of a journalist. They read Western newspapers, and then creatively went to work to make up their own stories, blending facts, fiction and half-truths, to be supplied to journalists in the West, agents of influence, or to be anonymously disseminated in Western countries.
According to the ex-Stasi intelligence chief Markus Wolf, Section X tended to develop a professional work ethic of its own that prided itself on fooling the West with the most realistic forgeries they could make. Language experts tried to mimic jargon of the people they tried to copy, and intelligence networks were used to steal real documents, alter them, and then bring them back into circulation in the West. Western printing equipment was smuggled to the East to be able to create exact replica's of Western (government) documents.
This particular operation of mailing neo-Nazi leaflets was probably encouraged by the KGB and happily seized upon by enthusiasts in Section X to get creative. It stood in a long tradition of the MfS using anti-semitism to discredit the West.
The reasons I can imagine in this specific instance for the MfS to mail such letters themselves is a combination of caution and vanity.
They were probably over-confident they could forge the stamps of West-German mail services to make it appear as if they had been really sent through regular mail. However, delivering the leaflets to the real West-German post offices risked exposing the agents of influence (and with it the MfS), and that the leaflets might be intercepted by the mail service itself for they contained forbidden materials.
The MfS thus opted to make its agents in the West deliver the letters directly personally, to the people who would likely take them to the media: Jewish Germans. And as expected, a scandal erupted and debates were had in the German parliament.
For further reading, I can recommend:
Jens Gieseke, "The History of the Stasi"
Markus Wolf, "Man without a face: the autobiography of Communism's greatest spymaster"
Michael Wolffsohn, "Die Deutschland Akte"
I also drew on some primary sources for this answer however.