Issue 9 of Email for a reason
The To recipients are the ones directly addressed. They are expected to be reached and possibly act on the email.
Often a recipient won’t notice whether they’re To or CC, so you can be extra clear and explicitly address the To recipients. For example: “Hi, Lisa, Peter, David and Molly!”
CC (Carbon Copy)
CC recipients are the ones who should be informed that the email has been sent, but are not expected to act on it, or are not directly concerned.
If a recipient doesn’t need to act, but it’s important that they are informed, use To.
BCC (Blind Carbon Copy)
BCC recipients get the email, but are not visible as recipients. Use this feature sparingly, if at all, since it creates a bad vibe to be secretive. The best use of BCC is when you are emailing lots of people, and don’t want to show the list of recipients. You simply email yourself and BCC all other recipients.