InvoiceShelf/app/Mail/EstimateViewedMail.php

<?php

namespace InvoiceShelf\Mail;

use Illuminate\Bus\Queueable;
use Illuminate\Mail\Mailable;
use Illuminate\Queue\SerializesModels;

class EstimateViewedMail extends Mailable
{
    use Queueable;
    use SerializesModels;

    public $data;

    /**
     * Create a new message instance.
     *
     * @return void
     */
    public function __construct($data)
    {
        $this->data = $data;
    }

    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
        return $this->from(config('mail.from.address'), config('mail.from.name'))
                    ->markdown('emails.viewed.estimate', ['data', $this->data]);
    }
}