How do I enable a user to link his Matlab license?

I have recently become a Matlab license administrator for my organization. I have a user who is trying to link his license to his Mathworks account, but is getting this error message -

The user has come to me as the error message suggested he do, but I do not know what to do for him. How does the license center know what email addresses are supposed to be valid? Was I supposed to provide a list of valid addresses in some way?