That's a good point, but I'd be surprised if Apple doesn't have (or is building) a mechanism to allow certain well-known domains to be trusted sender, in the ci

That's a good point, but I'd be surprised if Apple doesn't have (or is building) a mechanism to allow certain well-known domains to be trusted sender, in the circumstances you note. Like, have "enter your custom domain", but also "checkboxes for fedex.com, etc".