That's really strange. I have rented at least 30 times with Sixt over the last 5 years and they never sent me such a letter. However I'm pretty sure that one or two times I damaged a bumber or something under the car.

Sixt is actually a really reliable company in Germany. I know they use a lot of franchise outside Germany and they can be difficult. Hade my own experience in Norway where I got an old car with broken lights. Unfortunately the other rental car companies like Europcar, Hertz and Avis use a lot of franchise, too. And it is rather difficult to find out if it is owned or franchised.