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. A SAP can benefit customers who send large volumes of emails (such as 500,000 per month) and want to have consistent email branding with their domain (such as sender address, links, images, etc.). A SAP can also improve deliverability and avoid spam filters by proving that the sender is legitimate and trustworthy. Reference: https://help.salesforce.com/articleView?id=sf.mc_es_sender_authentication_package.htm&type=5