Include a button that would allow you to start a new email using a previously sent email's content.
Original post:
Nothing so maddening as hitting send and realizing something's missing/wrong/ changed. Being able to make an instant change at that moment so the corrected version lands right next to the erroneous one in the recipient's inbox. Minimizes irritation on the other end. But without that feature, it takes so long that it constitutes a more significant or numerous interruption. Instead of chuckling, the recipient grinds teeth and says unprintable things.