Upon even further consideration it looks like the From field is not of a prime interest either, so it too can go into the Details.
This cuts down the number of fields and renders section headers unnecessary, so they too can go.
Basically the idea is to reduce the dialog down to fields that actually need user's input and stash everything else away... I don't know if it's the right thing to do though. Perhaps having a summary of the whole email setup (including the subject, From and the message) is not such a bad thing. Not sure. Need to sleep on it.