Error communicating with target host

Copper Contributor

Currently I have this error message every time my users send mails to the other site "4.4.397 error communicating with the target host. -> 421.4.4.2 Connections were dropped due to a waiting period".
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.

Thank you
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