refactor

@@ -28,7 +28,7 @@ public class MailSender {
     List<Part> attachments;
 
     public static MailSender newMailSender() {
-        return newMailSender("smtp.126.com", "lxc1317@126.com", "luoxiuchun").from("UFO System<lxc1317@126.com>");
+        return newMailSender("smtp.exmail.qq.com", "no-reply@yoho.cn", "Xlcm@9646").from("UFO System<no-reply@yoho.cn>");
     }
 
     public static MailSender newMailSender(String host, String user, String password) {
@@ -137,7 +137,7 @@ public class MailSender {
     }
 
     @Builder
-    private class Part {
+    private static class Part {
         String filename;
         String content;
         String contentType;