The email is sent but the user name to email only. 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 to the email. 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 an e-mail to a single customer, do not know whether it came from the system on the test system to the system because it can create a user name.