Save it, don't send it. If you're writing a message and want to finish it later, tap Cancel, then tap Save Draft.

Finish a saved draft. Touch and hold Compose. Pick the draft from the list, then finish it up and send, or save it again as a draft.

Show draft messages from all of your accounts. While viewing the Mailboxes list, tap Edit, tap Add Mailbox, then turn on the All Drafts mailbox.

Delete a draft. In the Drafts list, swipe left across a draft, then tap Delete.