Save draft messages to send later
Mail automatically saves a message as you’re writing it, and when you quit Mail with an unsent message open. You can save a message as a draft yourself at any time.
For Exchange and IMAP accounts, drafts are stored on the mail server (unless you select a mailbox on your Mac in the Mailbox Behaviors pane in Mail preferences). As soon as Mail syncs with the mail server, your drafts are available wherever you use that account. To sync the drafts immediately, choose Mailbox > Synchronize > [account].
If you use Handoff, you can switch between your Mac and other devices while writing messages. To open a message handed off to your Mac, click the Handoff Mail icon that appears at the left side of the Dock.