Here is a note that I got back:
"There's different settings to determine the email address. If the authentication is done with something that looks like an email address, that will be used to send the expiration warning, otherwise you have the options below to choose from:
The last option, "Send a message to username@domain", assumes that the user provided just the "username" portion during the device enrollment, and the "domain" bit is supplied by the administrator in the "Unknown Domain" field.
IMO the easiest way to do this is to ensure that device enrollment is done based on email address and password - if that can't be done then the next best option is the username with a fixed domain."