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SoulwinnerFlag for United Arab Emirates

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451 4.4.0 primary target ip address responded with 421 4.2.1 unable to connect

MS exchange q Viewer gives this below message
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M A
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Please mention the version of Exchange.
Please make sure your DNS working properly.
Please configure external for external lookup.
ASKER CERTIFIED SOLUTION
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Belal Khallad

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@mas, exchange on Prem 2010 , we have O365 federation service
What SP / RU level are you at?  You can find this by running the following Exchange PS command.

Get-Command exsetup | foreach {$_.FileVersionInfo}

Assuming this was a working environment, what, if anything, has changed?  New ISP, new SmartHost, network equipment...etc..
This may or may not be related but review the article and check the NIC settings on the Exchange server...

https://oddytee.wordpress.com/2016/08/16/exchange-server-message-queue-error-451-4-4-0-dns-query-failed/
the problem was with the SMTP port 25 on the firewall. Now its sorted
Belal should be award the points.
Agree with Todd,
Belal cleary explained about the port 25 block to check in post
sorry guys :) thanks for your feedback