laravel 5.3 sendgrid categories

In laravel 5.2 this worked just fine, but since migrating to 5.3 i'm having issues getting the category sent in my email.

public function build()
    {
        return $this->view('mail.enquiry')
            ->getSwiftMessage()->getHeaders()->addTextHeader('X-SMTPAPI', json_encode(array("category" => array(env('BUSINESS_NAME')))))
            ->subject('Website Enquiry')
            ->to(env('MAIL_DEFAULT_TO_EMAIL'), env('MAIL_DEFAULT_TO_NAME'))
            ->from(env('MAIL_DEFAULT_FROM_EMAIL'), env('MAIL_DEFAULT_FROM_NAME'))
            ->replyTo(\Request::get('email'), \Request::get('full_name'));
    }

i get this error

BadMethodCallException in Mailable.php line 525:
Method [getSwiftMessage] does not exist on mailable.

every thing in this code works fine, but breaks as soon as I add this line:

    ->getSwiftMessage()->getHeaders()->addTextHeader('X-SMTPAPI', json_encode(array("category" => array(env('BUSINESS_NAME')))))

via cardi777