Or, or... and bear with me here... or, packaged click-button solutions with paid (contractually obligated) dedicated product support is a better use of our short time, more often than not.
Not when it's down.
No hard data though. My mail server only ever went down when I upgraded the server and didn't check that everything was still working right away, or similar maintenance induced incidents. It never went down by itself.
Such systems only ever go down unpredictably on HW issues, or when overloaded/out of resources. Neither is very likely, because you're not trying to grow your service in any sense similar to VC backed enterprises. Most of the time it has constant very low load and resource use. And you can simply stop introducing changes to the system if you need more stability for some time. (stop updating, for example)