What makes it particularly brutal, to me, is that the failures are typically silent. If GMail starts spamboxing my emails, I don't get feedback about that. If I'm not getting emails, I don't get feedback about that.

You can script checking those, but you'll have to re-implement it for each provider. And there are some that you can't; if I'm applying for a job, they're not going to give me an email account to test whether my stuff is deliverable.

HTTP services are absurdly easy to monitor in comparison.

I’ve tried testing gmail and it’s very difficult to even get consistent results

In my experience, the brutality grows rapidly with the number of users. If it's just for you, and you have at least one alternative account, then it's not brutal at all.

A mailserver administrator is a sysadmin. Being a sysadmin is on the face of it unrewarding - nobody pats you on the back when everything is working normally. They only call you when there's a problem (and it's usually urgent, and sometimes critical, and they'd like to know who to blame). So if you run a mailserver with users, it starts to become a people job, and things begin to matter.

People certainly rely on email to a greater extent than other services - even mobile. I can order goods online without surrendering my mobile number, but I always have to provide an email address before I can complete the order.

But I've still enjoyed administering mailservers. Perhaps I liked the slight paranoia induced by trying to reconcile the boss's demands for functionality with his demands for security.

This 100%, e-mail is too critical and is not core to what my skills are, so I outsource it, even if I think it would be fun setting up initially.