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Exchange 2010 - 451 4.4.0 Primary target IP address responded with "421 4.4.2 connection dropped due to timeout"

During these days I had a major issue between two Exchange servers in different continents.

Suddenly email between two exchange servers wasn't delivered and I got this error:

 451 4.4.0 Primary target IP address responded with "421 4.4.2 connection dropped due to timeout"

here are actions that I followed to solve this kind of issue:

1. telnet on each exchange server on port 25 and trying to send email (I didn't got no errors but no email was delivered)

telnet smtpserver 25
helo Test
mail from: user1@mail1
rcpt to: user1@mail1


"End data with ."



2. Restart Transport services and Attendant

3. Restart servers

4. Enabled verbose logging on send and receive connectors

To locate relatives log we should open the Exchange Management Console, expand Server Configuration, click on the server role (which may be Hub, Mailbox or an Edge Server), and right click on the chosen server and Properties. Finally, click on Log Settings tab

And using trace32.exe to view logs and I found strange errors

5. So I suspected certificate problems so:

6. Get-ExchangeCertificate | fl

7. So I reattached same certificate to SMTP service, for your reference, here's the powershell command we ran to reattach itto the SMTP service:

Enable-ExchangeCertificate -Thumbprint xxxxxxxxxxx -Services SMTP

You can find the thumbprints for all availabe cert by running the Get-ExchangeCertificate command.

8. restarted attendand services and after some minutes everythings worked again

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