Issue #2223967 by AdamPS, jofitz, LKS90, sja112, shaktik, pminf, Berdir,...

   * Formats a message prior to sending.

   *

   * Allows to preprocess, format, and postprocess a mail message before it is

   * passed to the sending system. By default, all messages may contain HTML and

   * are converted to plain-text by the Drupal\Core\Mail\Plugin\Mail\PhpMail

   * implementation. For example, an alternative implementation could override

   * the default implementation and also sanitize the HTML for usage in a MIME-

   * encoded email, but still invoking the Drupal\Core\Mail\Plugin\Mail\PhpMail

   * implementation to generate an alternate plain-text version for sending.

   * passed to the sending system. The message body is received as an array of

   * lines that are either strings or objects implementing

   * \Drupal\Component\Render\MarkupInterface. It must be converted to the

   * format expected by mail() which is a single string that can be either

   * plain text or HTML. In the HTML case an alternate plain-text version can

   * be returned in $message['plain'].

   *

   * The conversion process consists of the following steps:

   * - If the output is HTML then convert any input line that is a string using

   *   \Drupal\Component\Utility\Html\Html::Escape().

   * - If the output is plain text then convert any input line that is markup

   *   using \Drupal\Core\Mail\MailFormatHelper::htmlToText().

   * - Join the input lines into a single string.

   * - Wrap long lines using \Drupal\Core\Mail\MailFormatHelper::wrapMail().

   *

   * @param array $message

   *   A message array, as described in hook_mail_alter().

use Drupal\Core\Entity\EntityInterface;

use Drupal\Core\Entity\EntityViewBuilder;

use Drupal\Core\Mail\MailFormatHelper;

use Drupal\Core\Render\Element;

/**

          $build[$key]['#label_display'] = 'hidden';

        }

      }

      $build['#post_render'][] = function ($html, array $elements) {

        return MailFormatHelper::htmlToText($html);

      };

    }

    return $build;

  }

use Drupal\Component\Render\FormattableMarkup;

use Drupal\Component\Render\PlainTextOutput;

use Drupal\Component\Utility\Html;

use Drupal\Core\Session\AccountInterface;

use Drupal\Core\Test\AssertMailTrait;

use Drupal\Tests\BrowserTestBase;

   *

   * @var array

   */

  public static $modules = ['contact', 'dblog'];

  public static $modules = ['contact', 'dblog', 'mail_html_test'];

  /**

   * {@inheritdoc}

    $this->assertRaw(new FormattableMarkup('@sender_name (@sender_email) sent @recipient_name an email.', $placeholders));

    // Ensure an unescaped version of the email does not exist anywhere.

    $this->assertNoRaw($this->webUser->getEmail());

    // Test HTML mails.

    $mail_config = $this->config('system.mail');

    $mail_config->set('interface.default', 'test_html_mail_collector');

    $mail_config->save();

    $this->drupalLogin($this->webUser);

    $message['message[0][value]'] = 'This <i>is</i> a more <b>specific</b> <sup>test</sup>, the emails are formatted now.';

    $message = $this->submitPersonalContact($this->contactUser, $message);

    // Assert mail content.

    $this->assertMailString('body', 'Hello ' . $variables['@recipient-name'], 1);

    $this->assertMailString('body', $this->webUser->getDisplayName(), 1);

    $this->assertMailString('body', Html::Escape($message['message[0][value]']), 1);

  }

  /**

   */

  protected function submitPersonalContact(AccountInterface $account, array $message = []) {

    $message += [

      'subject[0][value]' => $this->randomMachineName(16),

      'message[0][value]' => $this->randomMachineName(64),

      'subject[0][value]' => $this->randomMachineName(16) . '< " =+ >',

      'message[0][value]' => $this->randomMachineName(64) . '< " =+ >',

    ];

    $this->drupalPostForm('user/' . $account->id() . '/contact', $message, t('Send message'));

    return $message;