Deliberate and effective manipulation of addictive behavior is a real thing tech companies do, but IMO this "dopamine hit" language is so popular because 1. the word dopamine is sciencey and sounds good, plus it has "dope" in there, like the actual drug, and 2. the word hit also reminds people of hard drugs.
Similarly, all sensory inputs release neurotransmitters. It's how you perceive things, so all perceptions release them, all the way from the bottom raw inputs up through the fully processed semantic content and emotional reactions. You can't say something is bad merely because it causes reactions; you need to do a lot more work and distinguish why some reactions are good and others are bad, which at the very least requires an operational definition of "good" and "bad".
A new email is a source of novelty (it's new) and obtaining it requires little effort. So for the brain it's tastier than spending another block of time on some task that you've been working on and isn't new at all.