In the social mailer preprocess we set a header but this is dependent…

… on if a user can be found, this way it was set it will always return a boolean where it cannot find a user on

modules/social_features/social_mailer/social_mailer.module

@@ -11,6 +11,7 @@ use Drupal\Core\Link;
 use Drupal\file\Entity\File;
 use Drupal\file\FileInterface;
 use Drupal\social_mailer\Element\SocialMailerTextFormat;
+use Drupal\symfony_mailer\AddressInterface;
 use Drupal\user\UserInterface;
 use Drupal\symfony_mailer\EmailInterface;
 use TijsVerkoyen\CssToInlineStyles\CssToInlineStyles;
@@ -129,7 +130,7 @@ function social_mailer_preprocess_email(array &$variables): void {
   }
   else {
     $to = $email->getTo();
-    $mail = isset($to[0]) ?? FALSE;
+    $mail = isset($to[0]) ? _addressesToString($to)[0] : FALSE;
   }
 
   if ($mail && ($user = user_load_by_mail($mail)) instanceof UserInterface) {
@@ -165,6 +166,21 @@ function social_mailer_element_info_alter(array &$info): void {
   }
 }
 
+/**
+ * Render array of addresses as array of strings.
+ *
+ * @param \Drupal\symfony_mailer\AddressInterface[] $addresses
+ *   An array of Address objects.
+ *
+ * @return array
+ *   An array of strings.
+ */
+function _addressesToString(array $addresses): array {
+  return array_map(function (AddressInterface $address) {
+    return $address->getEmail();
+  }, $addresses);
+}
+
 /**
  * Implements hook_mailer_builder_info_alter().
  */