We just made a change to GMass. Before, if you specified a Cc or Bcc address, and then sent via GMass, the Cc and Bcc addresses were ignored and only the addresses in the To line received the email message.
Now, however, after user requests, we’ve added Cc and Bcc functionality such that any addresses specified as a Cc or Bcc will receive a copy of every individual sent email. For example, if you’re sending via GMass to five recipients, and you set a Cc address, the Cc address will receive all 5 emails.
In the example above, the email is set to send to the eight recipients in the To line. If sent via GMass, each of the eight recipients will receive an individual, personalized email, and the Cc address will receive all eight email messages as well.
Update February 2016: See our corresponding blog post on confusion between the To and Cc/Bcc fields.