So how do you find these emails? Unless there is some magic natural language processing trick that I haven't heard of, I'd doubt that a content-based happiness filter would work. Let's just do it by sender.
People remember how you make them feel. The best way might be to just show the user a list of her top contacts, and rate her contacts. Like rating songs in iTunes, but more fun.
The inbox will now only contain emails from people that make you happy. The stress-free workplace is born.