I wrote the email chapter for the book "Internet Secrets" in 2000, and I ran my own mail server 1997-2019, so I have a pretty good sense of what the landscape was like then versus now. QMail and Postfix were both out before 2001, so you didn't need to know sendmail at the time. You just had to be able to configure the mailer to get things up and running. And given that there were decent Linux distributions available, the technical challenge wasn't high.
The difference now is that from there, there's a lot more to understand if you actually want your email to get anywhere reliably. It's complicated, subtle, and much harder to resolve problems when you get it wrong. At the time, the biggest problem was bounces. Now deliverability has become a dark art.
Just out of curiosity, when did you start running your own mail server?