I was talking with a friend yesterday who asked me about "de-mail" and "gmx.de", both words I haven't heard in many years.
Apparently if you want to contact the pension insurance (Rentenversicherung) and you don't want to call them (Maybe because you don't speak german), or send a paper letter you'll have to sign up for this "secure email, but not really email" (https://en.wikipedia.org/wiki/De-Mail) service. It costs a small fee to set up (10 Euros), or a bit cheaper if you are listed in the "public de-mail phone book" (6 Euros) and you have to verify yourself through a government ID.
If you are not a german citizen you of course don't have this ID card and can't do the verification online, that means that you now have to go to one of the few places in the city where you can verify yourself for the service. Some of them are just small corner stores that usually also do postal services.
Very bizarre experience, just to get some simple answers related to your pension.