Besides Thunderbird, is there any good desktop mail client for Windows that doesn’t involve uploading mail to a cloud first?
They could already have access to your emails, because… you’re running their OS. They can slip in any code they want and run it with NT AUTHORITY\SYSTEM
-level privileges (comparable to root
-level privileges on Linux systems).
If you run any other OS you’ll also have to trivially trust the makers of that OS with root
-level privileges (or comparable).
(Personally I don’t believe that MS is scanning all your local emails, but they certainly have the technical possibilities to do so very trivially.)