451 4.4.397 Error communicating with target host. -> 421 4.2.1 Unable to connect -> SocketTimedout: Socket error code 10060'

Gavin Ross 101 Reputation points
2022-04-17T18:40:44.697+00:00

Hope someone can help. We have 4 Exchange 2016 Servers, in 2 separate DAGS. Out of no where, one of the Exchange Servers will not connect to the smart host. The 3 other servers are fine. When checking the queue on the server, the error is:

451 4.4.397 Error communicating with target host. -> 421 4.2.1 Unable to connect -> SocketTimedout: Socket error code 10060'

I have restarted the server multiple times, I have verified DNS, made sure nothing is getting blocked on the Firewall and still will not send. Does anyone have any suggestions on where else to look?

Exchange | Exchange Server | Management
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  1. Andy David - MVP 157.8K Reputation points MVP Volunteer Moderator
    2022-04-17T18:49:54.307+00:00

    What happens if you telnet on port 25 to the smart host from that Exchange Server?
    Does it connect?

    If not, then I would look at the smarthost itself if there is no firewall between them.

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  2. Gavin Ross 101 Reputation points
    2022-04-17T19:18:23.947+00:00

    Tried to telnet and that failed, however the firewall rule shows the traffic passed and was allowed. Tested from one of the other servers and had no problem connecting. They all use the same Firewall Rule?


  3. FabioRamirez 0 Reputation points
    2023-01-11T15:17:44.3033333+00:00

    Hi.

    This other article refers to a similar issue and could be of help to validate the smtpd_recipient_limit value

    [https://success.trendmicro.com/dcx/s/solution/1106080-451-4-4-0-primary-target-ip-address-responded-with-421-4-4-2-connection-dropped-error-message?language=en_US

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  4. Md. Abdul Razzak 10 Reputation points
    2023-09-26T07:03:52.7+00:00

    Check telnet on port 25 to the smart host from the Exchange Server. I believe it will help you to resolve the issue.

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