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

Posted on 2016-10-12
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Last Modified: 2016-10-16
MS exchange q Viewer gives this below message
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Question by:saul2paul
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LVL 25

Expert Comment

by:-MAS
ID: 41839917
Please mention the version of Exchange.
Please make sure your DNS working properly.
Please configure external for external lookup.
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Accepted Solution

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Belal Khallad earned 500 total points
ID: 41839926
The possible causes for the “451 4.4.0 Primary target ip address responded with: "421 4.2.1 Unable to connect." are:

 

1. Your hub transport server or edge server are unable to resolve the external domain names. It can be caused by the incorrect DNS sever settings.

2. Your firewall or ISP block the outbound traffics on port 25.

3. The IP address of your hub transport server or edge server are blocked by the target domain or other issues on the target domain.

The FQDN on the Receive Connector doesn't matter, that is inbound email only. It is only the FQDN on the send connector that is an issue. Ideally the FQDN on the send connector should match the PTR your ISP has set on your external IP address
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Author Comment

by:saul2paul
ID: 41840018
@mas, exchange on Prem 2010 , we have O365 federation service
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LVL 30

Expert Comment

by:Scott C
ID: 41840071
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..
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LVL 15

Expert Comment

by:Todd Nelson
ID: 41841361
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/
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LVL 3

Author Comment

by:saul2paul
ID: 41845627
the problem was with the SMTP port 25 on the firewall. Now its sorted
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LVL 15

Expert Comment

by:Todd Nelson
ID: 41845637
Belal should be award the points.
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LVL 25

Expert Comment

by:-MAS
ID: 41845638
Agree with Todd,
Belal cleary explained about the port 25 block to check in post
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LVL 3

Author Comment

by:saul2paul
ID: 41845641
sorry guys :) thanks for your feedback
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