diff --git a/core/lib/Drupal/Core/Mail/MailInterface.php b/core/lib/Drupal/Core/Mail/MailInterface.php
index fbde1b86dc76ceebe2e01e78f90b9dfef16a7829..910cfa9133d37073f3a94aeab525467a0dc3bc41 100644
--- a/core/lib/Drupal/Core/Mail/MailInterface.php
+++ b/core/lib/Drupal/Core/Mail/MailInterface.php
@@ -15,12 +15,20 @@ interface MailInterface {
    * 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().
diff --git a/core/modules/contact/src/MessageViewBuilder.php b/core/modules/contact/src/MessageViewBuilder.php
index 5155341d80dde047a5acc9393ac8b66cd8cfd7ea..4aeb2499940e142a7e4c968c43e631a6fdb923d4 100644
--- a/core/modules/contact/src/MessageViewBuilder.php
+++ b/core/modules/contact/src/MessageViewBuilder.php
@@ -4,7 +4,6 @@
 
 use Drupal\Core\Entity\EntityInterface;
 use Drupal\Core\Entity\EntityViewBuilder;
-use Drupal\Core\Mail\MailFormatHelper;
 use Drupal\Core\Render\Element;
 
 /**
@@ -40,9 +39,6 @@ public function view(EntityInterface $entity, $view_mode = 'full', $langcode = N
           $build[$key]['#label_display'] = 'hidden';
         }
       }
-      $build['#post_render'][] = function ($html, array $elements) {
-        return MailFormatHelper::htmlToText($html);
-      };
     }
     return $build;
   }
diff --git a/core/modules/contact/tests/src/Functional/ContactPersonalTest.php b/core/modules/contact/tests/src/Functional/ContactPersonalTest.php
index da19685e17e7bad3ef26da980371fd13ff719d63..4814c22a2a39793f0d2a7a976967c43d10f792c4 100644
--- a/core/modules/contact/tests/src/Functional/ContactPersonalTest.php
+++ b/core/modules/contact/tests/src/Functional/ContactPersonalTest.php
@@ -4,6 +4,7 @@
 
 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;
@@ -25,7 +26,7 @@ class ContactPersonalTest extends BrowserTestBase {
    *
    * @var array
    */
-  protected static $modules = ['contact', 'dblog'];
+  protected static $modules = ['contact', 'dblog', 'mail_html_test'];
 
   /**
    * {@inheritdoc}
@@ -116,6 +117,20 @@ public function testSendPersonalContactMessage() {
     $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);
   }
 
   /**
@@ -326,8 +341,8 @@ protected function checkContactAccess($response, $contact_value = NULL) {
    */
   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;