https://www.tb.pro/en-US/thundermail/
Hosted Securely in Germany
Your emails are protected by strict EU privacy laws and hosted on infrastructure you can trust. With servers located in Germany, Thundermail prioritizes your privacy while ensuring reliable, fast delivery worldwide.Looks like it's still owned by Mozilla/MZLA and thus subject to US jurisdiction.
I don't see how it's different from Amazon or Microsoft datacenters in the EU, which are not safe from the US government. As long as the US parent company can somehow get at the data, it is obligated to do so when a US agency asks for it.