User email invalid

Hi! I am a long standing paying customer and Admin of the account. I am having difficulty changing one of the account User's Primary email on file that was incorrectly entered when his account was set up. Each time I attempt to update the User's email I receive a pop-up that the User email is invalid. It is a company email address that is being used and this email is not being used for any other BOX account that the User may have independently. Please advise at your soonest availability. A customer contact number to discuss would be greatly appreciated.