Error communicating with target host exchange 2016

Sera 1

Hello,

Currently, I have this error message every time my users send mails to the other site "4.4.397 error communicating with target host. -> 421.4.4.2 connection dropped due to Timeout".
Here is my architecture: on my site I have 2 exchange 2016 servers (a and b) in cu 22 with a dag. The site opposite has the same architecture and is part of the same domain. To send it outside my site, all emails go through my server A and for the site opposite, they go through server C. For reception, the emails all go through my A, which redirects them to the main servers. sites. If I can summarize the problem: internal mail domain nok, mail to the outside ok, return mail from the outside nok.
I checked my dns, the servers are well informed as mx. When server A restarts, the emails pass immediately and then hang.

Thank you

Kael Yao-MSFT 37,496 Reputation points Microsoft Vendor

2022-01-24T01:47:24.37+00:00

Hi,

Please note that currently the Microsoft Q&A forum only supports communicating in English.
Would you please edit your question into English so we can better understand the situation?
Thanks for your understanding.

According to the result of machine translation, the issue occurs when sending to a specific external Exchange organization.
As the error message "4.4.397 erreur communicating with target host. -> 421.4.4.2 connection dropped due to Timeout" indicates there are problems with connection between your Exchange server and the target Exchange server, I suppose this issue may be due to network problems.

Since currently you have no problems sending to other recipients except this organization, if possible please contact them to check if there are some firewall settings blocking network traffic.
Kael Yao-MSFT 37,496 Reputation points Microsoft Vendor

2022-01-27T08:10:56.807+00:00

Hi @Sera

I am writing here to confirm with you how thing going now?
Did the issue get resolved?
Sera 1 Reputation point

2022-01-27T08:28:02.85+00:00

Hello, thank you for your answer. The problem is that I disabled the firewall on both sides. Some messages to pass others not, I thought that this may be due to mails blocking because of the xlsm files but I do not see what parameter to modify on the exchange management shell.
Kael Yao-MSFT 37,496 Reputation points Microsoft Vendor

2022-01-27T09:26:55.72+00:00

Hi,

Does this issue only occur with emails which have attachments, or emails which have a larger size (10MB for example) than the normal emails?
Sera 1 Reputation point

2022-01-27T09:32:15.173+00:00

Thanks for consider my problem.

No, some 6kb messages are blocked and all .xlsm files are blocked. I have to do manual redirection on the shell with the redirect-message command to forward
Kael Yao-MSFT 37,496 Reputation points Microsoft Vendor

2022-02-01T09:01:21.487+00:00

Thanks for the information.

How about external senders(outlook.com, gmail, etc.) sending xlsm attachments to Exchange servers? Would the emails also have this issue?

And in your main post, I suppose server A and server B should be Exchange servers. Is server C a smarthost or something else?
Kael Yao-MSFT 37,496 Reputation points Microsoft Vendor

2022-02-07T08:42:12.243+00:00

Hi @Sera

I am writing here to confirm with you how thing going now?
Please let us know if you would like further assistance.
Sera 1 Reputation point

2022-02-08T07:57:36.757+00:00

Hello, regarding the problem of xlsm files I do not have a smarthost server. The files manage to get out of the domain. But inside the domain the xlsm are blocked. Then concerning my problem on the socket error, I set up a Microsoft Exchange Server ACS Certificate. The error to change now it says 421 4.4.2 connection dropped due to timeout. I have to restart the det servers for the day, the emails go through without any problem then the next day I have the same problem. thank you for helping me.
Kael Yao-MSFT 37,496 Reputation points Microsoft Vendor

2022-02-09T08:43:59.34+00:00

Hi,
Can the two Exchange servers resolve each other corretly in internal DNS?
I found the following link with the similar issue which may be helpful: Improperly configured DNS causes internal mail to hairpin via firewall
(Please Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)