Office 365 Distribution Group Delivery Management

I have distribution groups that were migrated from on-premise Exchange org to O365 a few months back and it seems the default Delivery Management setting for migrations is "Senders inside and outside of my organization". The setting seems to be taken form the "not set" setting in the groups' AD attribute "MsExchRequireAuthToSendTo". So I change this to True in ADUC AttribEditor so only Internal senders can send to it. We have a dirsync setup between AD and O365.
However, there is one distribution group that I need to have all internal and only one external contact to be able to send to, but can't figure how to do that besides using that "Restrict who can send messages to this group" list in Delivery Management and adding individual senders, which can;t be external unless I have them as a contact in our organization.