Send later, beside Send when you write a message, sends it at a later time: Tomorrow morning, Tomorrow afternoon, Monday morning, or a date and time you choose, in your time zone. The message waits in Scheduled, a new folder between Drafts and Sent, and moves to Sent once it has gone. You do not need to be signed in when it is sent.
Open a message in Scheduled to see when it will be sent. Unschedule puts it back in Drafts to change it, the clock changes the time, and Move to Trash stops it being sent. If a message cannot be sent at its time, it goes back to Drafts with the reason, and a message from Magpie in Inbox says so.
Also fixed: pressing Enter in the subject or an address line of a message being written discarded the draft. Enter now does nothing there.