求救，寄至客戶端被退信

[misa]

各位大哥，請教一下，寄給客戶的郵件（其他同一網域的客戶都正常，只有此帳號的有異常）！
都會收到此被退信的郵件，可否請教該如何解決！謝謝！
Reporting-MTA: dns; mail-sqr.daily.com.tw
Received-From-MTA: DNS; [192.168.2.46]
Arrival-Date: Thu, 21 Dec 2006 09:00:35 +0800 (CST)

Final-Recipient: RFC822; [email protected]
Action: failed
Status: 5.5.0
Remote-MTA: DNS; mx.sina.net
Diagnostic-Code: SMTP; 554 ironport3_200.sina.net
Last-Attempt-Date: Thu, 21 Dec 2006 09:00:44 +0800 (CST)