The outgoing box may hold emails which are too large for Outlook to handle, and it generates this cryptic message.
The following steps can help when Outlook is hung up on a large outgoing email:
On the menu, select File, then Work Offline.
It may list multiple items. Focus on emails that the column "In Folder" shows for Outbox.
It is likely the email in the outbox is the cause of the problem. Select the item(s) and right-click and select Delete. It will confirm you wish to delete the message, so select Yes.
Exit the dialogs and on the menu, select File, then Work Offline to switch back online.
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