laravel 5.4 在邮件中嵌入图像

I have just upgraded my 5.2 install of laravel to 5.3 and then to 5.4 following the official upgrading methods.

I am now trying to use one of the new features, to create a markdown formated email.

According to the documentation found at: https://laravel.com/docs/5.4/mail#view-data

要嵌入内嵌图像，请在$message上使用嵌入方法

However, this:

<img src="{{ $message->embed(public_path().'/img/official_logo.png') }}">

将产生以下错误:

未定义变量:message

我错过什么了吗？或者升级指南中是否有未记录的内容？

稍后编辑:

I am calling the email function with:

\Mail::to($user)->send(new WelcomeCandidate($user, $request->input('password')));

候选人看起来像:

<?php

namespace App\Mail;

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

use App\Models\User;

class WelcomeCandidate extends Mailable
{

    use Queueable, SerializesModels;

    public $user;
    public $password;

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

    /**
     * Build the message.
     *
     * @return $this
     */
    public function build()
    {
        $this->subject('Welcome!');
        return $this->markdown('emails.welcome-candidate');
    }
}