A Sender Authentication Package (SAP) is a set of features that authenticate an email sender's identity and reputation, such as a private domain, a dedicated IP address, a link customization, and a reply mail management. By having an additional SAP for the new domain comms.nto.com.au, NTO can send emails from their new domain without impacting deliverability. An additional IP address is not required if they already have one for their SAP. An additional private domain is not enough to authenticate their sender identity and reputation. Registering the new domain in From Address Management is not necessary if they have an SAP for the new domain. Reference: https://help.salesforce.com/articleView?id=sf.mc_es_sender_authentication_package.htm&type=5 https://help.salesforce.com/articleView?id=sf.mc_es_from_address_management.htm&type=5