I use a silicone nozzle cover to hide my shame.

Traitor? To what country?

If the Yanks could stop stealing credit for our Aussie anti-drone bogans, that’d be great.

Have you tried setting it in settings>apps>all apps>proton mail>open by default?

Edit: I install proton to test. I turned off open supported links for the Gmail app. I got the prompt you posted and hit always for proton. Mailto links now open with proton.