With Mirror Replication Agent 12.6 ESD#10, you do not need use sp_adduser to add the Maintenance User to the primary database when you set up the Maintenance User login. Instead, use sp_addlogin to create a server login account for the Maintenance User in the primary data server.
Versions earlier than Mirror Replication Agent 12.6 ESD #10
require adding the primary database Maintenance User login to the
primary database.
See Chapter 2 in the Mirror Activator Administration Guide for more information about setting up the primary database Maintenance User.