[pkz]

A large email provider like Gmail could probably train a network to find emails that look similar to an official brand (start with payment providers). The client could warn the user that "Warning! This email looks a lot like it is trying to impersonate a different brand".

[tsimionescu]

That's exactly what spam filters are. Since they are relying on heuristics and ML, they aren't completely accurate, unfortunately.

It is fair to criticize some of the obvious misses though - these algorithms and heuristics are clearly pretty bad.

[noAnswer]

Since a view years United Internet (mail.com, gmx.de, web.de) verifies some big players. So a email from Paypal, eBay or Postbank is visibly different from a standard email. (In the Web and App UI. Of course not if you use IMAP.) IIRC it has a individual icon and a green boarder.

(I don't know whether there is a standard behind it or if it is some kind of manual certificate pinning.)

[shswkna]

The paid-for “Google Workspace” does mark emails that look suspicious. It also uses the well known Gmail interface.