Lately, I've hardly been getting any Nagios notifications whatsoever. Being the cocky network admin that I am, I thought it was due to my rock solid infrastructure. Unfortunately, that is not true.
I found a few hosts and services that have notifications listed for them in the notification history page. I never received these messages.
I started to poke around in my postfix log and noticed that they never showed up there either.
I sent a test email from a terminal session, I got it. I sent a test email using the command that Nagios uses to send mail, I got it.
I restarted my Nagios box and then stopped a service that is being monitored. I got the notification. I started the service. I got the notification.
I edited my config to set a host to an IP address that doesn't exist. Nagios says it sent the notification, but I never got it. It never showed up in my postfix logs.