At the moment I have reports of two users that intermittently have emails stuck in their Outbox. When they noticed and get the emails sent the messages are not always saved to the Sent Items folder. Even when I check in OWA only some are in sent items. I confirmed the emails were sent through our 3rd party cloud filtering portal
One user also reports auto-complete does not work too around the time of the issue occurring. I can't see any stand out event logs errors that could cause this.
I could re-create their Outlook profile but why would the messages be sent but not saved to Sent Items even in OWA?