mail forwarding loop

[Tacobrother]

Hello everyone,

Two months ago, some letters be return by unknow reason. Below is the return mail and log

-----Original Message-----
From: Mail Delivery System [mailto:MAILER-DAEMON@foryou.com.tw]
Sent: Wednesday, November 2, 2016 10:17 PM
To: xxx@yyy.com.tw
Subject: Undelivered Mail Returned to Sender

This is the mail system at host mail.foryou.com.tw.

I'm sorry to have to inform you that your message could not be delivered to one or more recipients. It's attached below.

For further assistance, please send mail to postmaster.

If you do so, please include this problem report. You can delete your own text from the attached returned message.

The mail system

<otrs@yyy.com.tw>: mail forwarding loop for otrs@yyy.com.tw
--------------------log---------------------
Reporting-MTA: dns; mail.foryou.com.tw
X-Postfix-Queue-ID: 5E5CE60149
X-Postfix-Sender: rfc822; xxx@yyy.com.tw
Arrival-Date: Wed, 2 Nov 2016 22:17:07 +0800 (CST)

Final-Recipient: rfc822; otrs@yyy.com.tw
Original-Recipient: rfc822;otrs@yyy.com.tw
Action: failed
Status: 5.4.6
Diagnostic-Code: X-Postfix; mail forwarding loop for otrs@yyy.com.tw
------------------------------------------------
mail.foryou.com.tw is a aliases of otrs@yyy.com.tw If the problem is alias, client mail would not be affected.But they actually do.
Could someone help me ?
Thanks a lot.