Track Registrations as well as Receipts and Coupons.

Like receipts and coupons, I imagine one kind of email that a lot of OI accounts will get is the registration email: "Welcome to Generic Service! Your user name is Bens_really_long_but_guaranteed_unique_user_name. Blah blah blah." I will sometimes want to refer back to those and, while they will frequently be the first email in a given folder on OI, it would be nice if OI would recognize and categorize them the same way it will with coupons and receipts.