Mail (Yosemite): Save draft messages
Save a draft: Close the message window, then click Save in the dialog that appears.
Open a draft to continue working: Open the Drafts mailbox (from the Favorites bar or the Mail sidebar), then double-click the draft to open it in the message window. Now you can finish and send your message.
For Exchange and IMAP accounts, drafts are stored on the mail server (unless you deselect the option for your account 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.