Zen Cart Coupon Single User has been upgraded so that you can now restrict a coupon to any email address on a single domain, not just a single email.
So for example, if you want to permit a coupon to only be used by customers with a “@BigClient.com” email address, you would create a coupon code starting with “@BigClient.com”, such as “@BigClient.com_save_ten” or “@BigClient.com-half-off”.