missing scheduled send later mails in the sent folder

I have the same issue, and it was my very first time using Send Later, so it was very confusing if my mail actually got sent. Luckily my recipient took action from the email, otherwise I would have wondered if it ever got sent!

To detail my case, my computer was in sleep mode at the time the email was supposed to send. Hours later, when I woke my computer up, I could see the "Send Later" mailbox still showed "1", but when I clicked on it, the box was actually empty. The Sent box also did not show this email. My computer was awake for over an hour, yet this scheduled email was still invisibly stuck in the Send Later box. I decided to quit Mail and reopen, and then that email was NO WHERE: not in Sent, Send Later, Draft, it was just gone. Luckily a few minutes later my recipient took the action my email was requesting, so I believe it was not actually sent until I quit Mail and reopened to find the email missing.

This is a terrible user experience, especially when I assume a lot of people only schedule emails that are IMPORTANT to receive at a specific time. So not only is the received timing wrong, but I was left thinking I might have to spend a lot of time rewriting a delicately worded email and disturbing the recipient a second time about a delicate subject. You can't mess with people's important business like this!