Service Accounts (Configure Database Mirroring Security Wizard)
When using Windows Authentication, if the server instances use different accounts, specify the service accounts for SQL Server. These service accounts must all be domain accounts (in the same or trusted domains).
If all the server instances use the same domain account or use certificate-based authentication, leave the fields blank. Simply click Finish, and the wizard automatically configures the accounts based on the account of the current wizard.
If the database mirroring endpoints of the server instances are configured to use certificates, you must leave the service account fields empty.
To configure database mirroring by using SQL Server Management Studio