render alert mailer is not set up

resource/locales/en_US/admin/admin.json

@@ -27,6 +27,7 @@
     "attach_enable": "You can attach files other than image files if you enable this option.",
     "update": "Update",
     "mail_settings": "E-mail Settings",
+    "mailer_not_set_up": "The current mail setting is invalid.",
     "from_e-mail_address": "From e-mail address",
     "send_test_email": "Send a test-email",
     "success_to_send_test_email": "Success to send a test-email",

resource/locales/ja_JP/admin/admin.json

@@ -27,6 +27,7 @@
     "attach_enable": "許可をしている場合、画像以外のファイルをページに添付可能になります。",
     "update": "更新",
     "mail_settings": "メールの設定",
+    "mailer_not_set_up": "メールサーバーがセットアップされていません。",
     "from_e-mail_address": "Fromアドレス",
     "send_test_email": "テストメールを送信",
     "success_to_send_test_email": "テストメールを送信しました。",

resource/locales/zh_CN/admin/admin.json

@@ -27,7 +27,8 @@
 		"attach_enable": "如果启用此选项，则可以附加图像文件以外的文件。",
 		"update": "更新",
 		"mail_settings": "邮件设置",
-		"from_e-mail_address": "From e-mail address",
+    "mailer_not_set_up": "The current mail setting is invalid.",
+    "from_e-mail_address": "From e-mail address",
     "send_test_email": "Send a test-email",
     "success_to_send_test_email": "Success to send a test-email",
     "smtp_settings": "SMTP 设置",

src/client/js/components/Admin/App/MailSetting.jsx

@@ -61,6 +61,9 @@ class MailSetting extends React.Component {
 
     return (
       <React.Fragment>
+        {!adminAppContainer.state.isMailSetup && (
+          <p className="alert alert-danger"><i className="icon-exclamation"></i> {t('admin:app_setting.mailer_not_set_up')}</p>
+        )}
         <div className="row form-group mb-5">
           <label className="col-md-3 col-form-label text-left">{t('admin:app_setting.from_e-mail_address')}</label>
           <div className="col-md-6">