Because this email will be sent to the user name. Customers do not know what to make of that system. Because the system is tested on a real system is built using the same username.
Because this e-mail is sent but the user name. Make the customer does not know where it comes from the system in a test environment with real systems have the same user name.
Because this e-mail will be sent to the user name, but they do not know what to do if it had been in the system from the system test the system because I have created a user name like real.