> Things that have happened in the last 20 years include SPF, DKIM, DMARC

Right, of course. The protocols are more complex. (Add TLS, MTA-STS...) But whereas 20 years ago you _had_ to start from scratch and understand the whole stack, today that's just not necessary. There are numerous projects that make running a mail server readily possible without knowing e.g. the sendmail configuration macro language. And there are many many more good resources to learn it all if you want to know than there were 20 years ago. It is both a more complex technology and also undeniably easier for people to actually do it.