Although I am not an expert in this field, I believe that a law or an executive order could be passed requiring that if an external entity (Fastmail) wishes to engage in business with a US entity (server provider), end-to-end encryption must be enabled with a backdoor.
Additionally, when an individual pays Fastmail, they are essentially making payment to the server provider located in the United States.
US based?
https://purelymail.com/about