A common usecase for the need of open relays are MFC’s (”Copiers”) because people need to scan documents and want to mail them directly to recipients on the internet.
Never apply the following to your normal connector for mailflow, as your mailserver will most certainly be used to send spam!
Create a new receive connector (Server configuration, Hub Transport) and make sure it’s bindings don’t conflict with other connectors. The name for the connector could be something like “Relay Connector MFC’s” and make sure you add the right ip-addresses or ranges.
To grant the relay access, the following line will do the trick:
Get-ReceiveConnector "Relay Connector MFC's" | Add-ADPermission -User "NT AUTHORITY\ANONYMOUS LOGON" -ExtendedRights "Ms-Exch-SMTP-Accept-Any-Recipient"