I played with this a little bit today, and it was awesome. Not sure the tabs are needed, do you think more this page's information will continue to grow?

There's at least a bit more stuff coming, yeah (at a minimum: exposing mail tags more clearly, adding Herald routing information).

I also imagine this may start looking and working a little more like your Gmail inbox eventually, where some users might read some of their mail directly from the web UI, so I'm trying to bend it in that direction a bit. I think the primary use case for this UI will be "understand what happened to mail", at least for a while, but I'd like to see if "read mail" can also eventually be a major use case. Stuff like "mail headers" is a little bit helpful in understanding what happened to mail, but totally useless when reading mail, which is why I moved it to a separate tab and tried to make the main view look about as much like an inbox message as it can.