The UPS monitoring server goes through the firewall to reach the SMTP relay server. After hardening, the test emails are not going out. The server engineer was pulling his hair out because everything looks fine at his end. There is simply dead air, there was no response from the monitoring server and no test email. By right this would have been escalated to the SMTP team. But they were doing UAT with the customer, so I took a look.
Using my notebook to stand in as the UPS, my telnet to port 25 was unresponsive. A quick check in the SMTP settings for hosts allowed to relay, IP address was not included. Added a bunch in. This time it went through. Restore back the cabling, the UPS now reports authentication failure. On a hunch, I checked the Authentication Settings. Surprise, surprise. It was set to Integrated Windows Authentication. Highest available but that only works for Windows operating system. Checked Basic Authentication and the test mails went through.
By the book going by the towers would have taken another day.