7,925 questions
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.
451 4.4.397 Error communicating with target host. -> 421 4.2.1 Unable to connect -> SocketTimedout: Socket error code 10060'
![](data:image/svg+xml, %3Csvg xmlns='http://www.w3.org/2000/svg' height='64' class='font-weight-bold' style='font: 600 30.11764705882353px "SegoeUI", Arial' width='64'%3E%3Ccircle fill='hsl(57.599999999999994, 23%, 15%)' cx='32' cy='32' r='32' /%3E%3Ctext x='50%25' y='55%25' dominant-baseline='middle' text-anchor='middle' fill='%23FFF' %3EGR%3C/text%3E%3C/svg%3E)
Gavin Ross
101
Reputation points
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
4 answers
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Andy David - MVP 157.8K Reputation points MVP Volunteer Moderator2022-04-17T18:49:54.307+00:00 -
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?
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Andy David - MVP 157.8K Reputation points MVP Volunteer Moderator
2022-04-17T19:48:11.303+00:00 Any logs on the smart host? Sure its still configured to accept connections from that server?
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Gavin Ross 101 Reputation points
2022-04-17T20:26:41.737+00:00 I would have to contact the vendor as we don't have access to the logs. We are NAT'ing the address, so they all come from the same one, so doubt that is the issue.
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Andy David - MVP 157.8K Reputation points MVP Volunteer Moderator
2022-04-18T00:04:22.233+00:00 If the firewall logs show its connecting to the smart host on port 25, and nothing has changed on your end, then its worth checking the smart host :)
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Gavin Ross 101 Reputation points
2022-04-18T19:42:49.547+00:00 We found the problem, was an entry in the FW that was causing the problem. Makes no sense why it was only affecting just the one server, but it working now!
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Kael Yao 37,746 Reputation points Moderator2022-04-19T05:33:45.19+00:00 Thanks for the update.
Glad to hear the issue has been resolved!
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FabioRamirez 0 Reputation points2023-01-11T15:17:44.3033333+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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![](data:image/svg+xml, %3Csvg xmlns='http://www.w3.org/2000/svg' height='64' class='font-weight-bold' style='font: 600 30.11764705882353px "SegoeUI", Arial' width='64'%3E%3Ccircle fill='hsl(150.4, 39%, 24%)' cx='32' cy='32' r='32' /%3E%3Ctext x='50%25' y='55%25' dominant-baseline='middle' text-anchor='middle' fill='%23FFF' %3EMA%3C/text%3E%3C/svg%3E)
Md. Abdul Razzak 10 Reputation points2023-09-26T07:03:52.7+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.