If you've ever sent an email attachment through Outlook 2016 that exceeds the allowable attachment size and you're on a Mac, you will end up with the dreaded "The total attachment size exceeds the limit. Try removing one or more attachments before saving or sending" error notification.
It will not go away until you hunt down the offending email. Unfortunately the Outbox is not visible by default. How do you find the email when it is not appearing in Outlook 2016 or OWA?