Nagios SMTP2 Error

I’ve been getting these incessant error emails every few hours for weeks now:

** PROBLEM Service Alert: is CRITICAL **

"connect to address and port 25: Connection refused "

It’s infuriating, I don’t understand what’s causing them. We do not host email on our Univention server, we just use a remote relay. We have no email apps or packages installed beyond the default postfix. Outgoing email is working fine since I’m getting these error emails reliably, constantly, every day, all the time, forever and ever… (do I sound bitter?)

There’s zero complaints about any email issues in mail.log or mail.err, everything is working as it should. It is correct that netstat -a does not show port 25 listed for localhost, but I’m not sure why that is or why I should care.