Smart Sending / Account Detection
I think it'd be great if Airmail could detect what account to send from by analyzing who you're emailing. For instance, if I'm emailing someone on my Exchange account, it'd be great if Airmail would automatically make the sender my exchange composer. Often times, I accidentally send emails from my personal email to work folks.
Another idea would be to ask which account to send from when you hit the compose button. That would be helpful as sometimes I forget to change the sender account manually.