When you don’t have time to finish typing an email on your iPhone, you can hit the Cancel button and save the email as a Draft to finish it later. What’s not obvious is where to actually find the email Draft once the window closes.
Your email Drafts are hidden under the Compose icon at the bottom right of the iPhone screen. Simply tap and hold the Compose icon to bring up a list of your email drafts, then tap on the one you want to continue typing.