Not sure if bug or a feature or a known server delay, sent 9 email at ~11:45 am and then got the 9x “email sent on your behalf” receipts at exactly 7 pm.
This happened to me as well. Sent an email some time yesterday, recieved the “you sent an email” message at exactly 16:00 today.
We recently changed the provider we use to send email, and that change broke certain types of emails. I fixed one issue on Friday and the second issue today, and sent out correct versions of all the emails that had been delayed. IIRC, the bug affected a total of about 105 messages, all of which should have now been sent out correctly.