One of our users is no longer able to see any mail in his "Sent Items" folder. When he sends out a message it sits in the Outbox until he manually selects "Send/Receive" from the menu. I checked the Outlook Admin Center and confirmed the messages are sending, but they do not appear under "Sent Items" on Desktop, Mobile (iOS), or Online.
I did confirm that he has the app configured to save sent mail, and his Windows edition does not have group policy settings. Microsoft says the Exchange service is up and no other users have reported an issue.
Can anyone provide guidance as to where I should look next?