That's the advantage of choosing Matrix: it is compatible with a multitude of clients and servers, so take your pick. No need to install the BundesMessenger frontend. No need to trust the government, how very un-German.
*most convenient. The best option is obviously coming in person, with a ring binder containing all relevant documents as well as written records of all previous communication
Be sure to queue up 2h before opening time of the office you want to visit because everyone else is also dropping by in person too and the office closes for public service at 12:00.