Add a mailto link in an email

I am following the directions to add a mailto link with a prefilled email to a message. I keep getting and error: "Please enter a valid email address (e.g., [email protected])" after following all of the directions. I think it is breaking when I try to add the additional cc emails: [email protected] as it seems to work when I take that part out. Has CC changed this feature so it doesn't work? Could it not be working because I have a basic accounts? Any information would be helpful.