Allow the send-from email field to be blank, forcing me to ensure I'm sending from the right address. Suddenly sending from a different email has caused a lot of confusion for both me and the recipient. If it is an impossible programing task to let me set the email to be used for each contact, then leaving the field blank when composing would be a huge help. I want all my email in one account, so I'm not logging in multiple times.
(note: I've updated this suggestion to reflect that it is only when beginning a new email, and not when responding.)