However, I have read that encryption remains if you stay within the same domain.
Proton to Proton.
And is transferred unencrypted when you change domains.
Proton to outlook, Proton to Comcast. etc.
A huge and rather complicated task — you can create your own email server. I have done this. It is more of a message service that uses the email app but it has encryption but doesn't cross domains and remains below the radar and is essentially free if you have a spare computer.
Email sent can be traced at the receiving end. If it hits the internet then it can be traced.