Laravel - Mail - Customizing The Swiftmailer Message

The withSwiftMessage method of the Mailable base class allows you to register a closure which will be invoked with the SwiftMailer message instance before sending the message. This gives you an opportunity to deeply customize the message before it is delivered:

     * Build the message.
     * @return $this
    public function build()
        $this->withSwiftMessage(function ($message) {
                'Custom-Header', 'Header Value'
        return $this;