Updating the OS... usually a fairly rare occurence again, kernel vulnerabilities that affect you with minimal services exposed are quite rare, most vulnerabilities are local escalations and such which wouldn't affect just a mail server. When was the last remote root exploit in the linux kernel? Maybe SSH which would be the only other thing you might run... Neither is within my memory. I suspect these would be ever less frequent than mail service updates, if ever.

As for the rest those aren't really issues you need to actively watch, you'll just know when something goes wrong to take a look. I can't see running out of disk with just a mail service, crashes are... never and compromise/DDoS isn't really a risk if you secure it properly to begin with and update it that once every few years. It's really very hands off if you're not running any other services and just review the vulnerabilities.

I'm guessing this is even less of a problem for most of us though because I and probably many others around here already run a personal dev server which is kept up to date regularly, so I was speaking simply to adding mail functionality to an existing system previously.