You will receive a No Such User bounceback message when you attempt to send a message to an email that does not exist. Until the administrator creates the email account, or you correct the email address (if mistyped), you will be unable to send them a message. Below is what a version of the bounceback could look like:
This message was created automatically by mail delivery software.
A message that you sent could not be delivered to one or more of its
recipients. This is a permanent error. The following address(es) failed:
test@test.com
No Such User Here