This article describes how to run Outlook in Safe Mode which is useful for troubleshooting mailflow and the performance of the program.

To start the application in Safe Mode, quit Outlook and navigate to Start > Run > type outlook.exe /safe in Open field and hit Enter or click OK. Outlook will start in Safe Mode.
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Safe Mode 
For more information on troubleshooting connectivity in Outlook refer to the artcile: How Do I Troubleshoot Connection Issues In Outlook?