As far as I can see on their website, they don’t end to end encryption or zero-knowledge encryption. If that is true, it means that they are able to read all your emails (and so can the government if they order them to reveal the data). They sometimes use some pretty confusing marketing slag in general. It’s misleading because they advertise things like in-transit TLS encryption, which is standard nowadays. Even Gmail, Outlook, iCloud, Yahoo and other mainstream email providers have this by default. This is nothing special and they hope that people think it means the same as E2EE. If you care about data ownership, you should also care about (end-to-end) encryption. Only when you are the only key holder, you can be sure that no one can access your private stuff.