If you send an e-mail to multiple recipients (whether it is in the TO, CC, or BCC fields), you will receive a notification that 'somebody' opened your e-mail.
Our tracking works by embedding a pixel into the bottom of your message. When the recipient opens the message, we capture the 'open event'. If there are multiple recipients, we cannot confirm which recipient (and associated email address) opened the message. However, we are able to send the IP address and location most commonly associated with the IP address. Using this information, you may be able to determine which recipient viewed your message.