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Exchange Server 2013 Troubleshooting: Emails queuing with error 421 4.4.2 Connection dropped due to SocketError

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Scenario

Creating a connector for a site and breaking them out directly to the ISP resulted in the following error:

441 4.4.1 Error encountered while communicating with primary target IP address:
“421 4.4.2 Connection dropped due to SocketError.”
Attempted failover to alternate host, but that did not succeed.
Either there are no alternate hosts, or delivery failed to all alternate hosts.
The last endpoint attempted was xx.xx.xx.xx:25

Troubleshooting Steps

  1. Checking DNS entries on the local server and verified they not pointing to an external address.
  2. Verify with the ISP that they are accepting mail from the public IP. All online tests pass with the ISP.
  3. Do a route print. Check that no static routes are defined for Exchange.
  4. Telnet to the ISP smart host on port 25. No errors encountered.
  5. Trace Route to the smart host - Problem found.

After doing the trace, it wasn't getting past the first hop. The problem was on the firewall and a route had to be adjusted. After that, the errors were gone.