This is pretty useful. A couple months ago I started making a free self-hosted version of Boomerang for Gmail, but lost interest because I felt like I would have to devote most of my time to gmail's idiosyncrasies. Maybe it's time to get back to work.

Is there a way to add navigation events (eg. user opened email) to the observe gmail.observe API? Right now it looks like I would have to poll gmail.check.is_inside_email.