Hi @Glenn Maxwell ,
Do you mean the user cannot send any messages out from neither OWA or the Outlook client?
If possible, could you remove all personal information involved and share a screenshot of the messages stuck in the Outbox?
Besides, the Get-MailboxFolderStatistics cmdlet can return hidden items, so the result may be different with the visible items count to the end user. That beings said, if there are not that many items stuck, and you are able to move/copy the stuck messages, it's suggested to move these items to another folder, then try it again to see if you can successfully send a new message out.
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