The form shows a red banner that reads Mail sending failed. Contact your host. The banner appears after the submit button stops spinning.
The message means WordPress was unable to deliver the email the form generated. The failure occurs inside the wp_mail function that CF7 relies on.
Typical triggers include a disabled PHP mail function, an invalid From address, missing email headers, incorrect SMTP settings, server mail quota limits, SPF or DKIM records that reject the sender, firewall rules that block outbound SMTP, custom code that throws an error during the send process, or an outdated CF7 version that does not match the server PHP version.
When any of these conditions exist the mail process aborts and CF7 reports the generic host message.