Save draft emails in Mail on Mac
Mail automatically saves email messages while you write them, and when you quit Mail with unsent messages open. You can save a message as a draft yourself at any time.
Save a draft
In the Mail app on your Mac, make sure you’re in the message that you want to save.
Choose File > Save.
You can also close the message window, then click Save in the dialog that appears.
Open a draft
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.