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EXCHANGE PROBLEM 451 4.4.0 Primary Target IP address responded 421 4.2.1 unable to connect


I have a Exchange Server 2010 on a Windows Server 2008 X64.
Everything was good but since once week i have message not delivery ton my client.
In the exchange manager console i can see this error:

451 4.4.0 Primary Target IP address responded 421 4.2.1 unable to connect. Attempted failover to the alternate host but That did not succeed. Either, there are ni alternate hosts, or delivery failed to all alternate hosts

I have an other send connector and i have the differents domains with trouble inside.
I used the

With some domains, i have no problem.

Could you help me?

NB: Sorry for my English I'm not so good --> (French Speaking)
Avatar of neilpage99
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I had that problem before.
For me it turned out to be poor DNS resolution. Make sure yoru Exchange server can successfully resolve the MX records (and they are the correct MX records) for your destination email server.

From the Exchange Server:
1. Open a command window.
2. type:

Open in new window

...and press <ENTER>

3. Type:
set q=mx

Open in new window

4. Type:

Open in new window

...but use the real domain name of the recipient that you're trying to send to.

5. Verify that your query returns valid results - compare them to what you find when using
Avatar of JUGGER97139


No problem with those commands

the return is valid
Did you use the link I provided, and enter the recipient domain in the MX and DNS tools to verify resolution is correct?

Can you send a test email to the SAME RECIPIENT from a different, external test email account like GMail or Yahoo?

Try this link: 
And enter information about BOTH your sending email server, and the recipient email server.

Also, are you using any third party resources, like a smart host, or forwarder, or special send connector?
If that doesn't help, consider the possibility that you need to for SMTP "HELO" commands instead of the native EHLO.
Yes i did

I can send a test email to the same recipient from external account

I used a send connector like a smart host  (

The resolution is correct but i have a dns request timed out
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The error you received is often a result of SPAM filter on the recipient side. They are denying your sending SMTP connection because it is registered on a RBL or some other spam-related list. I know you said you're using a smarthost connector, but confirm email is being sent using that connector, and that the provider on the other end of that connector is no having spam related issues.