Using exchange I would get an annoying ⚠️ icon in the status bar at the bottom. Clicking it led to a Sync Errors screen that had a list of errors for one item with error -19703 – Could not synchronize record. Clearing these errors was annoying have to click the trash can icon repeatedly. And yet more would appear. The simple fix is to find all instances of this (in my case it was a calendar event that had long been deleted), and delete them. Yet they still appeared.

The one way to finally get it to be removed for me was to delete the offending messages from my trash. Then and only then, was my daily nightmare over and I have not seen the ⚠️ icon since.

Software versions used at the time of writing: Microsoft Outlook 16.