Digital certificates are required for secure communications between on-premises %BRAND_EXCHANGE_ONLINE_SHORT% 2013 servers, ...

Digital certificates are required for secure communications between on-premises %BRAND_EXCHANGE_ONLINE_SHORT% 2013 servers, clients, and your %BRAND_OFFICE_365_SHORT% organization. You need to obtain a certificate from a third-party trusted certificate authority (CA) and then install it on Client Access and Edge Transport servers. We recommend that your certificate's common name match your primary SMTP mail domain (for example, contoso.com) for your organization.