ExternalInOutlook allow-list inputs and permissions
Review the inputs and permissions recorded for updating Exchange's ExternalInOutlook allow list with the companion script. AllowList.txt must be in the script's folder; the source reports testing only in VS Code with PowerShell 7.
Source-derived reference
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As new addresses are added or old ones removed it is necessary to update the Set-ExternalInOutlook -AllowList in Exchange Online.
Again this has to be done using Powershell and the Exchange Online Management module.
The following Powershell script can be used to Enable the ExternalInOutlook flag AND update the Allow List.
The author reports testing only with Visual Studio Code and PowerShell 7. ISE and PowerShell 5 were not established as supported in the source.
You must have the Exchange Online Management module installed, however the script checks for that and will install if not.
Save the code to a folder on your local computer.
Save the Allow List to a file called AllowList.txt in the same folder on your computer.
You must have Exchange Admin Permissions or PIM'ed to Global Administrator as the Set-ExternalInOutlook flag is a global setting.
The source describes the script reading AllowList.txt and applying its contents to the ExternalInOutlook allow list. This outcome has not been retested here.