Why do sent messages remain in my Apple Mail “Drafts” folder after being sent?
We’ve occasionally received reports of message copies remaining in the “Drafts” folder after being successfully sent with Apple Mail. This seems more likely to happen with large messages that have attachments.
Nobody’s mentioned this problem with other IMAP mail programs such as the excellent Mozilla Thunderbird, so it’s something outside of our control, unfortunately. Hopefully Apple will eventually fix it, although the problem has existed for years.