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451 4.4.0 DNS query failed

We setup 4 Exchange servers for redundancy, exch-mail01, exch-mail02, exch-cas01 and exch-cas02. Once for a while exch-cas02 server EMC may has "451 4.4.0 DNS query failed" error in the queue as shown below.

Here are the detail message:

Identity: EXCH-CAS02\8991\31231
Subject: RE: New POC Monitoring Form Submitted
Internet Message ID: <4BD7D6316265A7428FAF17D7558BE5FE016B5271@EXCH-MAIL01.chicagobotanic.org>
From Address: userA@chicagobotanic.org
Status: Ready
Size (KB): 20
Message Source Name: FromLocal
Source IP: 255.255.255.255
SCL: -1
Date Received: 10/4/2010 8:19:02 AM
Expiration Time: 10/6/2010 8:19:02 AM
Last Error: 400 4.4.7 Message delayed
Queue ID: EXCH-CAS02\8991
Recipients: userB@chicagobotanic

The DNS is setup correct and nslookup looks good as shown below: I have searched the internet and found some similar issue, but it is Exchange 2007 and some suggest to disable IPv6. How do you troubleshoot it?

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