Set-ExternalInOutlook [[-Identity] <OrganizationIdParameter>] [-AllowList <MultiValuedProperty>] [-Enabled <System.Boolean>] [<CommonParameters>]
If your organization already uses mail flow rules (also known as transport rules) to add text to the subject line of messages from external senders, you should disable those rules before you enable this feature to avoid duplication.
You need to be assigned permissions before you can run this cmdlet. Although this topic lists all parameters for the cmdlet, you may not have access to some parameters if they're not included in the permissions assigned to you. To find the permissions required to run any cmdlet or parameter in your organization, see Find the permissions required to run any Exchange cmdlet.
Set-ExternalInOutlook -Enabled $true