I recently had a ticket come in for another customer that had a load balanced setup between three hosts: host1, host2, and host3. Each host uses a server-side script to send e-mail via an external mail server.
While troubleshooting, we noticed host1 and host3 were working just fine with the e-mail script. Host2 was having difficulties. We checked the firewall on all three servers; they were all the same. There were no custom entries in the Windows hosts file.
I finally had the bright idea to run
route print in command prompt and voila! There was an extra default gateway for host2. After deleting that extra gateway, all three hosts were functioning as normal.
Although it was not DNS this time, routing is definitely important in network traffic (obviously).