Similar to to the idea "Always use hide-my-email aliases" (https://protonmail.uservoice.com/forums/284483-proton-mail/suggestions/49195577-always-use-hide-my-email-aliases), but also that it warns you that you are not sending an email from an alias.

So you can still email from your base protonmail email, but a popup warns you that you are sending an email from it.