The problem is that spam was/is so bad that extreme measures were taken to curb it. There are all kinds of invisible forces that you abutt that can be difficult to figure out, such as IP blacklists and the like. And even if you set everything up properly and host your email with a responsible host, Microsoft will still mark your mail as spam.
I host my own email server with Vultr on an OpenBSD VM using OpenSMTPD and Dovecot, relaying all outbound mail through SMTP2Go (their free tier more than meets my needs). I have all of the necessary DNS entries set to mark my mail as legit, and I sign all outgoing mail using strong 2048-bit RSA keys. Thus far, I'm able to send mail and not have it marked as spam (at least to everyone that I've corresponded with thus far). It was a lot of work to get to that point, but not terrible.
I host my own email server with Vultr on an OpenBSD VM using OpenSMTPD and Dovecot, relaying all outbound mail through SMTP2Go (their free tier more than meets my needs). I have all of the necessary DNS entries set to mark my mail as legit, and I sign all outgoing mail using strong 2048-bit RSA keys. Thus far, I'm able to send mail and not have it marked as spam (at least to everyone that I've corresponded with thus far). It was a lot of work to get to that point, but not terrible.