Laravel 7 swift dkim usage; on new versions of laravel 9^, swift Mailer is remplaced with Symfony Mailer

dkim.php

<?php

# this file is in `config/manualdkim.php`

return [
    'private_key' => env('DKIM_PRIVATE_KEY', storage_path('app/dkim/private_key.txt')),
    'selector' => env('DKIM_SELECTOR', 'default'),
    'domain' => env('DKIM_DOMAIN', null),
];

GeneralMail.php

<?php

#...
# app/Mail/ResetPassword.php
#...

class GeneralMail extends Mailable{
#...
  public function build()
  {
  $this->subject(__('General Mail'))
    ->view('mails.resetpassword')
    ->text('mails.resetpassword_plain');
  add_dkim_to_mailable($this); // this call the helper
  return $this;
  }
}

Helper.php

<?php 
#-...
# app/Helpers/Helper.php

if (!function_exists('add_dkim_to_mailable')) {
    /**
     * This function add a Dim sign to mailable entities.
     * @param Mailable $mailable the mailable entity
     * @return void 
     */
    function add_dkim_to_mailable(Illuminate\Mail\Mailable $mailable)
    {
        $mailable->withSwiftMessage(function (\Swift_Message $message) {
            // DKIM INFO
            $dkim_privatekey = config('manualdkim.private_key');
            $dkim_selector = config('manualdkim.selector');
            $dkim_domain = config('manualdkim.domain');
            if (
                empty($dkim_privatekey) &&
                (!file_exists($dkim_privatekey) &&
                    (empty($dkim_selector) && empty($dkim_domain)))
            ) {
                return;
            }
            $signer = new \Swift_Signers_DKIMSigner(file_get_contents($dkim_privatekey), $dkim_domain, $dkim_selector);
            $message->attachSigner($signer);
        });
    }
}

private_key.txt

# storage/app/dkim/private_key.txt
#here write the private key generated from the dkim support on your host
#or from this great tool: https://tools.socketlabs.com/dkim/generator
# in the host manage go to cpanel zone editor and search dkim._domainkey.you_domain.com or default._domainkey.you_domain.com
# CPANEL
# Email Deliverability > [press "Manage" button of select domain] > see the dkim menu