Earlier quoted context omitted.
You still have to update the OS or whatever you're running the mailserver on. You have to make sure it's not compromised nor DDosed, it doesn't run out of space, it doesn't crash or get stuck for whatever reason, etc etc. Again, Docker isn't magic.
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 le…
Oh yeah, me too, and tbh it's enough of a pain already without having to deal with mail, spam and the likes.
There is a reason that companies like Heroku exist and prosper; it's the same reason personal mail servers never saw mass-adoption, despite being one of the first things you could do on the internet. The anecdote that you find it easier than most doesn't change the reality of the matter.