One of the main reasons why a user would choose an on-premises ESA versus the CES solution is to have more control over the sensitive data that flows through the email system. With an on-premises ESA, the user can ensure that the data is stored and processed within their own network and data center, and that they comply with any regulatory or organizational requirements for data security and privacy. With a CES solution, the user would have to trust Cisco to handle the data in their cloud infrastructure, and to adhere to the service level agreements and security policies that are agreed upon. Some users may not be comfortable with this level of outsourcing, especially if they have strict data governance or compliance needs12. References: 1: Physical ESA vs Cloud ESA - Cisco Community 2: Cisco Email Security Appliance - Data Sheet