The reason you are not finding much information is because the issue is outside of the messaging gateway. The error is telling you that the Messaging Gateway was able to connect to the next hop and start a conversation, but the network connection was truncated by a source other than the Messaging Gateway. As per your research, this is often due to Firewalls or Intrusion Detection Systems affecting the network; however, it can also be due to malfunctioning network equipment.
You will want to research this at the network level. The best option is to log your network traffic between you and the end recipient and review those logs to determine the source of the issue.
Regards