Exchange 2010退信 Gmail 『554 5.1.0 Sender denied』

公司網域某個特定用戶(user1@domain.com.tw)寄給Gmail帳戶(user2@gmail.com)正常

但是(user2@gmail.com)回覆會被退信如下：
From: Mail Delivery Subsystem <mailer-daemon@googlemail.com>
Date: 2013/3/19
Subject: Delivery Status Notification (Failure)
To: user2@gmail.com

Delivery to the following recipient failed permanently:

user1@domain.com.tw

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain domain.com.tw by mail.domain.com.tw. [IP Address].

The error that the other server returned was:
554 5.1.0 Sender denied

Log 紀錄：
19T09:13:54.402Z,08CFF22805920108,[IP Address]:25,209.85.215.46:42562,209.85.215.46,<CAJaQfhjru2bpDGDkkGnJfWe2jM1VujG_wpmOE7oBY+bKgB0vSQ@mail.gmail.com>,user2@gmail.com,1,Content Filter Agent,OnEndOfData,AcceptMessage,,SCL,1

環境：單一Exchange2010+OfficeScan10，無他方過濾軟體，只有這個user2@gmail.com回覆到user2@gmail.com會被退信，其餘用戶正常
集線傳輸/已啟用的傳輸規則：
1.空白主旨之郵件
2.主旨/廣告過濾
集線傳輸/反垃圾郵件採預設值，檢查過正常

請問我該怎麼解決？郵件追蹤確實有以上Content Filter情況，可是我找不到。