Proper Email Address Etiquette
To, Cc and Bcc: What’s the Difference?
Most people know what happens when they address mail TO: people when creating a new message.
However, they are less sure about the two other options: CC: (carbon copy) and BCC: (blind carbon copy).
Use To: For Private Messages
If you are sending the message to only one address then the TO: option should be used.
Use BCC: for Lists
If you are sending the message to multiple addresses then the BCC: option should be used so that each person sees only their own address.
Use CC: Only in Special Cases
The CC: option essentially works the same way as the TO: option. Everybody will see the addresses included there. The main difference is intent.
In most cases you'll use the CC: option when you want to keep someone in the loop but no action is required AND all parties know the address. See the example below.
Source and details: https://www.russharvey.bc.ca/resources/email.html