Hello,
From the message tracking, although your MTA connected to ESA seems to be in the same network of 172.x.x.x, I would recommend you to check for any delays caused in the network between the two appliances.
You can run a packet capture on the appliance during communication to show the complete connection and handshake, and associated issues that may be causing the connection to be interrupted.
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If you need help getting the packet capture, kindly follow these steps (GUI):
Go to "Help and Support " >> Packet Capture.
Click on "edit settings."
Under "Filters" choose "Predefined Filters".
Type the sender's ip address in the "server ip" field.
Click "submit".
Click "start capture".
Ask the sender to send a test email
After finishing click "finish".
Select the capture and click "download file".
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I hope the above information might be helpful for you!
Cheers,
Pratham
