SOLUTION FOUND!
I found out that DNN 5.6.1 has a bug in the email validation regular expression.
The 5.6.1 regular expression for email validtion is:
^[a-zA-Z0-9_%\-+](?:\.*[a-zA-Z0-9_%\-+])*@[a-zA-Z0-9_\-](?:\.*[a-zA-Z0-9_\-])*\.[a-zA-Z]{2,9}$
The pre-5.6.1 regular expression validation is:
\b[a-zA-Z0-9._%\-+']+@[a-zA-Z0-9.\-]+\.[a-zA-Z]{2,4}\b
This can be found under Admin->User Accounts->User Settings->Email Address Validation
So this turned out to be a bug not in Registration Pro but in DNN.