Contact Form 7 checks each mail block before sending. It validates the Cc header in Mail (2). If the header does not meet the required format, the block is marked invalid.
The validation looks for a colon after Cc and a proper email address. It also verifies that any tag used returns a valid mailbox.
When the header fails any of these checks, CF7 stops the Mail (2) process. The primary mail may still be delivered, but the copy never leaves the server.
The error appears in the form editor and prevents the autoresponder from reaching the intended recipient.