When you are composing a new message in Mail app and then tap the “Cancel” button, there are options for you to delete the draft or to save the draft. Saved draft can be retrieved later to continue writing the message.
To access the saved drafts, tap the email account under “Accounts” section at the main Mail app screen, then tap on “Drafts”.
To quickly access the saved drafts, tap and hold the compose button on the main screen. Mail app will display a list of saved drafts. Simply tap to select the draft to continue message composing.