When CircleCI sends workflow-related emails it seems to use a BC: list rather than To: or CC: lists.
BC: is really a solution for low-value bulk email shots, rather than a communications solution in a business environment, and it causes a number of issues.
When I see a project-related message, I have no idea who else working on the same project has also seen the notification. This is made more complicated by the fact that anyone with an account can just unsubscribe from such messages without there being any way to gain a project-level oversight of such choices.
Without a real email address on the address line it is rather hard to use many email tools for message filtering. You can not tell if the email was addressed to an alias account or with many tools filter on the recipient’s email address as there is none.