We use Amazon Simple Email Service (SES) with locations in the EU for sending emails, for everything else we use dedicated servers at a provider headquartered in Germany (EU) or in Switzerland - depending on the location and preference of our customers.
The reason for using Amazon is that reliable email sending is demanding. And we have not found a convincing provider headquartered in Switzerland or the EU that offers email sending via a programming interface.
We are not satisfied ourselves that we use Amazon. For this reason, we are already working on our own email sending infrastructure. The goal is that we will use our own servers at local providers in Switzerland and the EU also for email sending. This work is still ongoing (as of August 2021). For this reason, we are still using Amazon for a transitional period.