virtualmin notification Mail Queue Size is down

I received this message from a virtualmin because I subscribed to notifications. A real thumbs up on notification in virtualmin to detect issues!

Today I got this message. I get it when . Isee sshd is down or httpd but this one isn’t really a daemon service. Is this part of the postfix multiple functions that happen? I know postfix has a lot of child processes.

Monitor on for ‘Mail Queue Size’ has detected that the service has gone down at 24/Apr/2019 10:05
Current status: Down - 15 messages queued

in the end this was due because the machine was out of live memory