~~Hmmm, that's true. Probably OP checked this, but now that you mentioned it, I would do the same 😁~~

Screw that, they were reading all the foreign mail coming through. And you could still be unable to see if they opened your envelope because they were either carefully opening it and close it back, or simply use another one.

What happens with ordinary email by the way?

I expect it was previously appealing based on the fact that only the vendor had access (of course, now it no longer applies).

Maybe another option is to use one of the private providers that do not track you and are less mainstream than Proton. Let's say Tuta or Mailbox.org (if they are not blocked already too). Probably encrypt the emails with PGP too for more security.