I get a "Can't add that email address" when trying to add a new user. I have sufficient licenses available.

I get a "Can't add that email address" when trying to add a new user. I have sufficient licenses available and was able to add other users subsequently without error.  I have 3 email address that returned this error message.  I tried to add each several times which produced the error message each time.  Logged out of the session and back in and the error message appeared each time.