Laravel: How to Render Mailer SMTP Request

How do I render the request being made to an email driver (e.g. SparkPost)? I need to debug a 403 issue and the error message is vague.

Context

MailController.php

Mail::send(new Generic($request));

Mail/Generic.php

public function build()
{
    return $this->to('thanos@galaxy.net')
        ->from('starlord@galaxy.net')
        ->subject('You Were Wrong')
        ->markdown('emails.generic');
}

via Chebli Mohamed