I think what he's trying to say is his downtime is planned - kernel upgrades, configuration changes, that sort of thing.
This is both somewhat true and somewhat not true. He's still vulnerable to failure of the hosting equipment and attacks from crackers. But, when he's the only admin, there's nobody else to fat-finger the server out of action, and if he only logs into the shell for service he will never fat-finger outside of planned service.
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.
I know what he meant, but it's not like google is improvising with gmail. The site is already up by the way, it's been years since they have a more than 5 minute downtime. Also, you can never dream of having the same kind of redundancy as gmail will have nor the collective man hours spent in the project. I don't hate the idea of self hosting your mail, I just think it's ridiculous to say you'll have a better service than gmail when you have much better arguments like privacy, control and ownership.
Availability covers data loss and account cancellations out of the blue, which has happened to a friend of mine at least. this is one of the reasons I self host.
Also, the machine is in the same lan as me so connectivity is never a problem (unless the cat chews through the ethernet cables again).
This is both somewhat true and somewhat not true. He's still vulnerable to failure of the hosting equipment and attacks from crackers. But, when he's the only admin, there's nobody else to fat-finger the server out of action, and if he only logs into the shell for service he will never fat-finger outside of planned service.