Contact Form 7 reports a failure for Mail (2) when the PHP mail function is not available. The plugin relies on WordPress’s wp_mail wrapper, which ultimately calls PHP’s mail().
When the server disables mail(), the call returns false. WordPress records the error and displays the message in the admin screen. No email is sent to the recipient or the site administrator.
The issue originates from the server environment, not from the form fields or shortcode. Shared hosting panels often turn off mail() for security reasons. Some hosts list mail in the disable_functions directive or omit a valid sendmail_path.
Other WordPress features using wp_mail, such as password reset emails, also stop working. The problem appears in the debug log as a PHP warning about mail() being disabled.