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Exchange 2003 SMTP service causes massive Internet slowdown

We have a single Exchange 2003 server (version 6.5.7638.1). It has been running great for months on end since being provisioned.

Just today we started noticing our internet connection was running extremely slow. To monitor this, I setup a constant ping to www.google.com. We were getting frequent timeouts and response times of 500ms or worse. Normal response time is 12-16ms (no timeouts). This was on our cable internet connection. We switched over to our DSL connection. It started off really fast, but then it slowed way down too! 500ms or worse, with lots of timeouts.

Finally, we started unplugging individual machines from our network switch until the problem went away. We isolated it to one of our VMware ESX servers, then further down to our Exchange server virtual machine. After that, we finally isolated it to the SMTP service on the Exchange server. If the service was started, the network latency problem was in full effect. The second you stop the SMTP service, everything's fine again.

The queues are basically empty (there's a dozen or so messages going out), so our first suspicion of there being some kind of spamming going on proved false. We're also not getting an inordinate amount of incoming mail. In fact, almost all spam gets filtered by Postini before it even hits our SMTP server.

Network utilization on the Exchange server, while the SMTP service is running and the problem is being exhibited is less than 1%. CPU usage is below 5%. The server is virus/malware/spyware free.

We've stopped all outbound email just to see if that would help. It did not. We setup a filter to block all incoming email to see if that would help. It did not.

We are thoroughly stumped. Thanks
Email ProtocolsExchangeVMware

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8/22/2022 - Mon