Apart from the message body, every email message includes the so-called envelope and message headers. In fact, both the envelope and the headers contain the information about the sender and the recipients of the message, and they do not always have to match.
The difference between the two sets is as follows. The recipient addresses specified in the envelope (using the RCPT TO command) are the actual addresses that the sender wants the email to be delivered to. Upon delivery of the message, each recipient won't be able to figure out what other addresses it was meant to be delivered to. The addresses specified in the To message header however, are all included with the email message, and everyone receiving the message sees that same header in full.