I’ve never understood why you needed to hit Cancel on an email in order to save it as a draft. At least it's easy to access those drafts. Instead of backing out to your list of inboxes, selecting the inbox you want and then hitting Drafts, you can access those not-quite-ready messages with a single action. Simply long press the Compose button in the bottom right corner of the screen when viewing your list of messages. Boom, all your drafts for that account will appear.