Let's suppose I frequently exchange emails with a friend named Ed Winter, who used email address <firstname.lastname@example.org>.
One day I receive an email sent by Ed Winter <email@example.com>
When looking at that email, I immediately recognize the shown name, and I believe I'm certain who has sent that email. I don't notice that it was sent from an unusual email address.
If an email from a new email address is received, which isn't in the address book yet, and the display name matches an existing entry in the address book (which has a different email address), then show a warning.
The intention is to make the user aware that something unusual might be happening.
The warning could say something like: "Message from Ed Winter was sent from an unexpected email address. Your contact might be using a new email address, or it might be a different person with the same name, or it might be an attempt to trick you."
This is just an initial thought. I haven't fully thought this through, but I believe this kind of notice could be useful.
(I had already reported this idea at https://bugzilla.mozilla.org/show_bug.cgi?id=1844082 but I haven't received any feedback yet for this idea.)