TestHtmlMailCollector.php

<?php

namespace Drupal\mail_html_test\Plugin\Mail;

use Drupal\Core\Mail\Attribute\Mail;
use Drupal\Core\Mail\MailFormatHelper;
use Drupal\Core\Mail\Plugin\Mail\TestMailCollector;
use Drupal\Core\StringTranslation\TranslatableMarkup;

/**
 * Defines a mail backend that captures sent HTML messages in the state system.
 *
 * This class is for running tests or for development and does not convert HTML
 * to plaintext.
 */
class TestHtmlMailCollector extends TestMailCollector {
    
    /**
     * {@inheritdoc}
     */
    public function format(array $message) {
        // Join the body array into one string.
        $message['body'] = implode(PHP_EOL, $message['body']);
        // Wrap the mail body for sending.
        $message['body'] = MailFormatHelper::wrapMail($message['body']);
        return $message;
    }

}