Hosted my own email for 18 years, same experience, switched to pobox.com about two years ago. It really burns me up, but I need my emails to arrive where I sent them.

Yep I did the same. I ran my own email server off the same IP address for 15 years with zero problems, but I had to give up recently because email is not something that you can do on you own anymore. You now have to pay some somebody to vouch for you or else MS will send your emails to spam or even worse just randomly delete them.

Something should eventually come along and replace email. Like bitcoin and other technologies, what was originally designed to be decentralized became centralized in practice.

Bitcoin's history involves a plan for combating spam—that's why proof of work was invented.

Hashcash, Bitcoin's PoW game, was intended for use as an email attachment to create a real cost to spam without any micropayments.

"If you want to mail me, prove that you burned five seconds of CPU time."

That is really interesting. Do you know of any articles that go into more details?

Could be you had a customer/other recipient like my ex-boss: I'm fairly certain he used the spam button instead of delete on his Google apps for your domain (now gsuite I guess) account.