We allow domains to send mass email as long as the conform to the CAN SPAM act of 2003. You can see the details here: http://business.ftc.gov/documents/bus61-can-spam-act-compliance-guide-business.
Also, all user sign-up forms must use a CAPTCHA to prevent abuse. We can install your CAPTCHA for you for a reasonable fee. Details of CAPTCHA can be found here: http://www.captcha.net/.
