[dev] Hwo to make customer user email address optional when create ticket

I use OTRS to track phone support tickets w/o the need to send notification to customer. So I don't need the email address of the customers. However, OTRS require unique email address per each customer user when a customer user is added. How do I change this behavior? A config file or perl source, may be? Any hint for me to get started would be very grateful. Thanks, samphan.