source/Core/Email.php
public function headerLine($name, $value)
    {
        if (stripos($name, 'X-') !== false) {
            return null;
        }

        return parent::headerLine($name, $value);
    }
public function headerLine($name, $value)
    {
        if (stripos($name, 'X-') !== false) {
            return null;
        }

        return parent::headerLine($name, $value);
    }
public function headerLine ( $ name , $ value ) { if ( stripos ( $ name , 'X-' ) !== false ) { return null ; } return parent :: headerLine ( $ name , $ value ) ; }
Inherited phpMailer function adding a header to email message.
We override it to skip X-Mailer header.

@param string $name  header name
@param string $value header value

@return string|null
Inherited phpMailer function adding a header to email message . We override it to skip X - Mailer header .