SimpleLogin in practice - what am I missing?

While I am generally cautious about my privacy, up until now I have not gone too far into this rabbit hole.

I would like to change this. As a first step, I would like to change all my email adresses at various services and use a combination of SimpleLogin + Protonmail here. I just started working on this, and 3 out of 5 services denied the SimpleLogin mail (including all variants SL is offering).

Is this just bad luck or …

practice privacy