If it is a technical impediment, please reconsider this feature because the tech keeps changing with new iOS releases.
It is not acceptable to force your paying customers to use several mail apps on iOS. It should be the users choice if they want to use the built-in app or Proton Mail. Proton won't keep up with iOS features in terms of integration with the OS (and new stuff like AI). So please allow us to use the email clients we desire.

Thanks