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.)