Actually, you have a bug that has been unfixed for 5 years now. I know because I submitted it. Still no action.
Here's my last message to Proton Mail support, request ID 822331. I was directly told no resources would be spent on fixing it:
> Well, it has been multiple years now so can you guys maybe prioritize this? How long do you want me to continue waiting on this issue? I can't count on PM users to send my mailserver E2EE mail when the mobile app doesn't support it.