title: Only allow 'business' email address register on website
body: <p>We are looking to build a new website that is specifically for commercial/business use.  We need to only allow 'business' email addresses register.  So users with Gmail, Hotmail, Yahoo etc cannot apply with those email addresses.   </p>
  
  <p>Is there a way to do this, other than creating a blacklist of domains such as gmail.com, hotmail.com etc?  It will be an international site - so it will be impossible to blacklist all of the public email providers.</p>
  
  <p>Thanks
  Ian</p>