I think the idea is that dopamine is involved in rewarding novelty seeking behavior. (https://www.sciencedaily.com/releases/2006/08/060826180547.htm https://www

I think the idea is that dopamine is involved in rewarding novelty seeking behavior. (https://www.sciencedaily.com/releases/2006/08/060826180547.htm https://www.sciencedaily.com/releases/2006/08/060826180547.h...) 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.