Hi @Larbi Messarat ,
The error message points out the user or group not found exception.
I checked the official document, and it should be -User "NT AUTHORITY\ANONYMOUS LOGON"
Try this:
Get-ReceiveConnector "Anonymous Relay_73" | Add-ADPermission -User "NT AUTHORITY\ANONYMOUS LOGON" -ExtendedRights "Ms-Exch-SMTP-Accept-Any-Recipient"
Best regards,
Lou
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