diff --git a/core/core.api.php b/core/core.api.php
index cf3b3353ea69..27bc96d5c0f1 100644
--- a/core/core.api.php
+++ b/core/core.api.php
@@ -1947,9 +1947,10 @@ function hook_queue_info_alter(&$queues) {
  *     Subject of the email to be sent. This must not contain any newline
  *     characters, or the email may not be sent properly.
  *  - 'body':
- *     An array of strings containing the message text. The message body is
- *     created by concatenating the individual array strings into a single text
- *     string using "\n\n" as a separator.
+ *     An array of strings or objects that implement
+ *     \Drupal\Component\Render\MarkupInterface containing the message text. The
+ *     message body is created by concatenating the individual array strings
+ *     into a single text string using "\n\n" as a separator.
  *  - 'headers':
  *     Associative array containing mail headers, such as From, Sender,
  *     MIME-Version, Content-Type, etc.
@@ -1998,7 +1999,9 @@ function hook_mail_alter(&$message) {
  *     string when the hook is invoked.
  *   - body: An array of lines containing the message to be sent. Drupal will
  *     format the correct line endings for you. MailManagerInterface->mail()
- *     sets this to an empty array when the hook is invoked.
+ *     sets this to an empty array when the hook is invoked. The array may
+ *     contain either strings or objects implementing
+ *     \Drupal\Component\Render\MarkupInterface.
  *   - from: The address the message will be marked as being from, which is
  *     set by MailManagerInterface->mail() to either a custom address or the
  *     site-wide default email address when the hook is invoked.
diff --git a/core/modules/contact/contact.module b/core/modules/contact/contact.module
index 63e1c4d8d048..7cba6b13e69f 100644
--- a/core/modules/contact/contact.module
+++ b/core/modules/contact/contact.module
@@ -135,7 +135,7 @@ function contact_mail($key, &$message, $params) {
       $message['subject'] .= t('[@form] @subject', $variables, $options);
       $message['body'][] = t("@sender-name (@sender-url) sent a message using the contact form at @form-url.", $variables, $options);
       $build = entity_view($contact_message, 'mail');
-      $message['body'][] = (string) \Drupal::service('renderer')->renderPlain($build);
+      $message['body'][] = \Drupal::service('renderer')->renderPlain($build);
       break;
 
     case 'page_autoreply':
@@ -154,7 +154,7 @@ function contact_mail($key, &$message, $params) {
       $message['body'][] = t("@sender-name (@sender-url) has sent you a message via your contact form at @site-name.", $variables, $options);
       $message['body'][] = t("If you don't want to receive such emails, you can change your settings at @recipient-edit-url.", $variables, $options);
       $build = entity_view($contact_message, 'mail');
-      $message['body'][] = (string) \Drupal::service('renderer')->renderPlain($build);
+      $message['body'][] = \Drupal::service('renderer')->renderPlain($build);
       break;
   }
 }