If a sender never emailed me before, the UI could display a tooltip or an icon alerting me as such.

How would this help? If someone from npmjs.help is emailing me about a 2-factor authentication update, if I am alerted that it is the first time that email address is sending me emails, I am going to think twice before clicking it.

Important! Let's make it the first time they are emailing me in my inbox (in case they sent an email with the purpose of arriving at my junk folder and, a few months later, send another email that arrives in my inbox, which would bypass this system).