CC and BCC
In order to save all emails you put as CC or BCC in Redmine communication, go to Administration -> Helpdesk, General tab and check in the following option:
When you reply to an email, by default the answer will be sent to all contacts - TO (the original address only), CCs and BCCs from the original message. If you click on pencil icon next to Send note field you may clean or change CCs and BCCs.
There is one more thing connected with cc & bcc. If the latest comment in the ticket was received from someone in CCs then their e-mail will be automatically set as TO e-mail when you will answer to ticket.