smtplib in 2.7 doesn't know anything about RFC822 or any of the replacement RFCs.  sendmail accepts a *string*, and doesn't understand or modify anything about that string except the newlines.  It is your responsibility not to *add* the BCC header.  What you want to do is put the BCC (and CC!) recipients in your toaddr list passed to sendmail, and *not* add a BCC header.

In Python3 smtplib has a send_message method that accepts a Message object, and that method uses the BCC to inform where to send the message and strips the header before sending. That is, smtplib's send_message method *does* implement RFC5322 behaviors.