In standard Bugzilla the bugmail header X-Bugzilla-Reason: CC could mean that you are on the C list or you are watching someone on the CC list. Bugmails from bugzilla.gnome.org have a patch to differentiate between those. X-Bugzilla-Reason: WatchCC is used when watching someone on the CC list and X-B-R: CC is only used when you are on the CC list.
In 2.20 the email code has been changed drastically. Fortunately above could be reimplemented. A new feature is the X-Bugzilla-Watching header. It is a comma separated list of email addresses you are watching for this bugreport.
It doesn’t work entirely like I’d want to. It can lists email addresses that have not caused the email to be sent. E.g. you are watching some person who has voted for a bug. In your email preferences you’ve disabled receiving bugmails when voting. Although the X-Bugzilla-Reason wouldn’t specify WatchVoter, the X-Bugzilla-Watching will specify the voter. This only happens when you receive the mail due to some other reason (example: you are the assignee). I’m not planning to fix this.