Hardware breakages and Zero-Day exploits usually tend to show up at most inconvenient times. I'd recommend everybody to not own an email server from a pure business perspective, unless you're a mail server admin anyways. There's so much time you can sink in it and still not achieve the level of security and uptime a quality email host will reach for much less money.

Now, there are arguments to run your own mailserver - you get to configure it exactly the way you want, you might see it as a hobby, ... But from a pure financial point of view it doesn't make sense.