The problem with email is that identity and authentication are an afterthought. Don't forget that (in theory) it is possible to get any email server to relay a message for you. Newer protocols do not have these kind of problems.
That would be an open relay. That is simply not something that mail servers do anymore. If one was to deliberately set up an open relay, one would find that their email server was blacklisted pretty much immediately.
I self-host my mail for over 17 years. Most of the spam I'm observing those days comes from hacked/broken websites (sometimes it's probably some stolen SMTP credentials, sometimes sent from the server directly). Legit domain name, SPF and even DKIM present, looks totally legit in this regard - only stopped by RBLs and content filtering.
I remember back in the day having to change your SMTP settings whenever you travelled to whatever the ISP was where you were staying. then you could finally send email from your @homeisp.example email
Store and forward.
Do remember that email was THE great federated protocol.
The goal of a mail server was to get your email "at least one hop" closer to your destination. And that wasn't an easy task.
Servers came online and went offline. Users logged in and out. Connections came up and went down. IP wasn't the only transit. DNS? Oh, the hosts file? Even higher things--thing DECnet and Janet.
Email was barely functional most days. Your best bet if you weren't an Internet God and weren't able to write your own super complicated sendmail.cf was to know a sysadmin at a node who had an Internet God and ask him if you could forward emails that you couldn't handle to their server.
Unfortunately, greedy people soon jumped in to take advantage of this generosity, resulting in a tragedy of the commons.
John Gilmore used to run an open relay, and I used to get spam from it. He was really stubborn about promoting the freedom of the spammers over the peace and quiet of the poor recipients. He eventually got shut down, still complaining.
Because the people on the 'net were generally not bad actors back in the day, so we would you need to lock things down?