Do you mean mailto links? Mail is not a type of app but a type of link. Depending on where you’re opening the link from, sometimes you can long press to recall the open with menu and choose always open with proton. If that doesn’t work check the app setting on the default apps section, there’s a “link opening” sub menu. There you can see which apps open which internet links. Sometimes a link is not a generic mail to but a specific gmail link that is configured to automatically open with the app. Maybe try disabling gmail from opening links altogether or try to find the offending link on the messy link list. There’s a lot of dead google services that are now redirected to gmail.