Contact Form 7 validates every header before sending mail. The Bcc header must follow strict RFC syntax. When the header contains an empty value, stray characters, or a malformed address, the validator rejects the entire Mail (2) request. The error appears as a red banner on the front end and stops both primary and secondary messages.
The validator checks for a single colon, a space, and a comma‑separated list of valid email addresses. Any deviation, such as missing angle brackets, extra spaces, line breaks, or a form tag that does not return an email, triggers the failure. The plugin then returns the message “Invalid mail (2) – Invalid ‘Bcc’ address”.
Server‑side mail agents also enforce header correctness. If the Bcc line reaches the MTA with invalid syntax, the MTA rejects the message and CF7 reports the same error. This behavior protects against header injection and spam.