Improving laravel mails to outlook/hotmail

I have a Laravel app sending emails via MailGun. For Gmail addresses it works right of the bat, but for Outlook/Hotmail they are marked as spam and have broken formatting. I have identified some issues but not sure how to best fix them.

I am aware this post is broad but the common thing is how to please outlook/hotmail.

Multipart messages

According to MailGun help:

It is best to send multi-part emails using both text and HTML or text only. Sending HTML the only email is not well received by ESPs.

Given my Mailables' build function below, how can I achieve that?

public function build()
{
    return $this->from('name@mysite.com')
        ->subject('Welcome to mysite!')
        ->view('emails.welcome_new_user', [
            'updateLink' => env('APP_URL') . '/subscribers/' . $this->subscriber->id . "/edit/" . $this->subscriber->token,
            'deleteLink' => env('APP_URL') . '/subscribers/' . $this->subscriber->id . "/delete/" . $this->subscriber->token
        ]);
}

Headers

According to MailGun help:

Make sure you are using unsubscribe links and headers in your emails. Many ESPs (particularly Hotmail) pay attention to this and if they are not there, you are likely to get filtered.

In the view I have two unsubscribe links, one as a button in the body and one in the footer as plain text link so that should be fine. But how do I supply Headers and which headers? I assume it is done similar to this section in Laravel docs:

$message->getHeaders()->addTextHeader('Custom-Header', 'HeaderValue');

Other tips on specific Outlook/Hotmail requirements

Also most appreciated