Contact Form 7 validates every email header before sending. The plugin checks the syntax of the From, To, Reply‑To and other address fields. If any address does not match the RFC‑5322 pattern, the form stops processing and shows an error.
The error appears when the address contains missing characters, extra spaces, stray symbols or when a form tag that does not return an email is placed in a header field. The validation also rejects addresses that do not belong to the site domain, because CF7 blocks cross‑domain senders by default.
When the validation fails, CF7 aborts the mail routine. No email is sent, no success message appears, and the submission is not stored in Flamingo or other logging plugins. The user sees a banner that reads “Invalid email address” or “Invalid mailbox syntax”.