Why can't users be BCC'd on tickets in Support?
Zendesk Support does not recognize BCC's and they are not supported in tickets.
If a user is added as a BCC on an inbound email, the user won't be added or acknowledged anywhere in the ticket or events. Additionally, they won't be notified of any ticket updates from the agent interface.
If the BCC'd user responds to the original email it may be captured through our system's inbound email threading processes causing their message to be appended to the existing ticket in the form of a private note.
For other resources, see the article: Using CCs, followers, and @mentions.