heroku環境下でLaravelのアプリケーションからメールを送りたい

やりたいことは何でしたか？
heroku環境下で新規登録成功時にメールを遅れるようにしたい。

起きている問題は何ですか？
※できるだけ詳細に記載しましょう
このエラーが出る前は500server errorになってしまった。 https://gyazo.com/e7219aa31d4570209842685d8bc0ea3f この画像のようなエラーが出てしまう。

エラーメッセージは何ですか？（テキストをコピー＆ぺーストするようにしましょう）
上記の画像のようなエラーが発生する。ローカル環境においては、正常にメールを送ることができる。

関連するソースコードは何ですか？（Githubのリポジトリリンクでも可）

mail.php

<?php return [ /* |-------------------------------------------------------------------------- | Mail Driver |-------------------------------------------------------------------------- | | Laravel supports both SMTP and PHP's "mail" function as drivers for the | sending of e-mail. You may specify which one you're using throughout | your application here. By default, Laravel is setup for SMTP mail. | | Supported: "smtp", "sendmail", "mailgun", "ses", | "postmark", "log", "array" | */ 'driver' => env('MAIL_DRIVER', 'smtp'), /* |-------------------------------------------------------------------------- | SMTP Host Address |-------------------------------------------------------------------------- | | Here you may provide the host address of the SMTP server used by your | applications. A default option is provided that is compatible with | the Mailgun mail service which will provide reliable deliveries. | */ 'host' => env('MAIL_HOST', 'smtp.mailgun.org'), /* |-------------------------------------------------------------------------- | SMTP Host Port |-------------------------------------------------------------------------- | | This is the SMTP port used by your application to deliver e-mails to | users of the application. Like the host we have set this value to | stay compatible with the Mailgun e-mail application by default. | */ 'port' => env('MAIL_PORT', 587), /* |-------------------------------------------------------------------------- | Global "From" Address |-------------------------------------------------------------------------- | | You may wish for all e-mails sent by your application to be sent from | the same address. Here, you may specify a name and address that is | used globally for all e-mails that are sent by your application. | */ 'from' => [ 'address' => env('MAIL_FROM_ADDRESS', 'hello@example.com'), 'name' => env('MAIL_FROM_NAME', 'Example'), ], /* |-------------------------------------------------------------------------- | E-Mail Encryption Protocol |-------------------------------------------------------------------------- | | Here you may specify the encryption protocol that should be used when | the application send e-mail messages. A sensible default using the | transport layer security protocol should provide great security. | */ 'encryption' => env('MAIL_ENCRYPTION', 'tls'), /* |-------------------------------------------------------------------------- | SMTP Server Username |-------------------------------------------------------------------------- | | If your SMTP server requires a username for authentication, you should | set it here. This will get used to authenticate with your server on | connection. You may also set the "password" value below this one. | */ 'username' => env('MAIL_USERNAME'), 'password' => env('MAIL_PASSWORD'), /* |-------------------------------------------------------------------------- | Sendmail System Path |-------------------------------------------------------------------------- | | When using the "sendmail" driver to send e-mails, we will need to know | the path to where Sendmail lives on this server. A default path has | been provided here, which will work well on most of your systems. | */ 'sendmail' => '/usr/sbin/sendmail -bs', /* |-------------------------------------------------------------------------- | Markdown Mail Settings |-------------------------------------------------------------------------- | | If you are using Markdown based email rendering, you may configure your | theme and component paths here, allowing you to customize the design | of the emails. Or, you may simply stick with the Laravel defaults! | */ 'markdown' => [ 'theme' => 'default', 'paths' => [ resource_path('views/vendor/mail'), ], ], /* |-------------------------------------------------------------------------- | Log Channel |-------------------------------------------------------------------------- | | If you are using the "log" driver, you may specify the logging channel | if you prefer to keep mail messages separate from other log entries | for simpler reading. Otherwise, the default channel will be used. | */ 'log_channel' => env('MAIL_LOG_CHANNEL'), ]; ContactSent.php <?php namespace App\Mail; use Illuminate\Bus\Queueable; use Illuminate\Contracts\Queue\ShouldQueue; use Illuminate\Mail\Mailable; use Illuminate\Queue\SerializesModels; class ContactSent extends Mailable { use Queueable, SerializesModels; protected $title; protected $text; /** * Create a new message instance. * * @return void */ public function __construct($name,$text) { $this->title = sprintf('%s様',$name); $this->text = $text; } /** * Build the message. * * @return $this */ // buildメソッドがviewを返している。これでviewを使って、簡単にメールの内容を扱える public function build() { return $this->view('email.sample_mail')->subject($this->title)->with(['text' => $this->text]); } } sample_mail.blade.php <!DOCTYPE html> <html lang="ja"> <style> body { background-color: #fffacd; } h1 { font-size: 16px; color: #ff6666; } #button { width: 200px; text-align: center; } #button a { padding: 10px 20px; display: block; border: 1px solid #2a88bd; background-color: #FFFFFF; color: #2a88bd; text-decoration: none; box-shadow: 2px 2px 3px #f5deb3; } #button a:hover { background-color: #2a88bd; color: #FFFFFF; } </style> <body> <p>{{$text}}</p></br> </br> <p>ご登録ありがとうございました</p> </body> </html> RegisterController <?php namespace App\Http\Controllers\Auth; use App\Http\Controllers\Controller; use App\Providers\RouteServiceProvider; use App\User; use App\Mail\ContactSent; use Illuminate\Support\Facades\Mail; use Illuminate\Foundation\Auth\RegistersUsers; use Illuminate\Support\Facades\Hash; use Illuminate\Support\Facades\Validator; class RegisterController extends Controller { /* |-------------------------------------------------------------------------- | Register Controller |-------------------------------------------------------------------------- | | This controller handles the registration of new users as well as their | validation and creation. By default this controller uses a trait to | provide this functionality without requiring any additional code. | */ use RegistersUsers; /** * Where to redirect users after registration. * * @var string */ protected $redirectTo = RouteServiceProvider::USER; /** * Create a new controller instance. * * @return void */ public function __construct() { $this->middleware('guest'); } /** * Get a validator for an incoming registration request. * * @param array $data * @return \Illuminate\Contracts\Validation\Validator */ protected function validator(array $data) { return Validator::make($data, [ 'name' => ['required', 'string', 'max:255'], 'email' => ['required', 'string', 'email', 'max:255', 'unique:users'], 'password' => ['required', 'string', 'min:8', 'confirmed'], ]); } /** * Create a new user instance after a valid registration. * * @param array $data * @return \App\User */ protected function create(array $data) { $mail_name = $data['name']; $mail_text = $mail_name.'様、ご登録ありがとうございました'; $mail_to = $data['email']; Mail::to($mail_to)->send( new ContactSent($mail_name,$mail_text)); return User::create([ 'name' => $data['name'], 'email' => $data['email'], 'password' => Hash::make($data['password']), ]); } } 試したことは何ですか？最初500sever errorが発生したので、 heroku plugins:install heroku-configというプラグインをインストールして、ローカルの.envファイルとherokuの環境変数を同期できるようにした。その後heroku config:pushによって、herokuの環境変数に.envを同期した。参考記事（https://e-tec-memo.herokuapp.com/article/277/）うまく行かない原因の仮説は何ですか？上手くいかない仮説はgmailの設定が何か間違っている？使用ライブラリ、ライブラリのバージョンなどの環境情報を教えてください。 Laravel Framework 6.18.0