I run mine on an Ubuntu LTS. About once every 2 weeks the server sends me an email about an update which just requires me to run apt update. I could even automate that if I wanted to.

If you're interested in running a linux VPS for mail/web/anything use webmin for general linux admin.

There is a learning curve involved in running linux, you can't get away from that, but that curve would have been substantially steeper without it.

To keep your system up to date with it is trivial..

(not connected, just a fanboy)

So, the downside is that you need to install Debian's unattended-upgrades?!

Yeah, you better do release upgrades manually--but that is once every few years.

In addition to debian's unattended upgrades there's Ubuntu's live kernel update mechanism that adds kernel to the list of things that are up-to-date.

qmail:

Latest Stable release: 1.03 / June 15, 1998

That one component out of hundreds didn’t receive any updates for a long time doesn’t really make a point.

And the last release of qmail patches to support IPv6, SPF, dkim, and a few other useful things which do not come included?