In outlook when you "reply to" a message or "reply all" to a message, it captures who the email was from and who the email was to, including CC fields. If you add new emails in the cc: or message field, it would not capture the added recipients information. I'm not familiar with Zimbra email. You may want to inquire on their website or support number regarding capturing email headers in a reply. I actually did a lot of searching this weekend to find any sort of workaround, for any client, and had no luck. Headers where hidden to remove exactly the type of information you want to capture