The mail routine stops because the Subject line does not meet RFC‑2822 rules. An empty value, stray line‑breaks, or illegal characters break the header format. CF7 validates the final header before sending. If validation fails, the mail is marked as invalid and no email is dispatched.
Dynamic tags often cause the problem. A tag such as [your-subject] that returns no value leaves the Subject line blank. The same occurs when a tag includes characters like \r, \n, or a colon. The parser treats these as header delimiters and rejects the message.
Additional Headers may also interfere. A malformed Subject entry placed in the Additional Headers field overwrites the correct value and produces the same error. The result is a red notice on the front end and no entry in Flamingo.
Server mail filters sometimes reject messages with empty or malformed subjects. The host records the rejection as an invalid header error, which CF7 reports to the user.