I used to run my own e-mail server for my personal address. In an attempt to reduce spam I configured Postfix to reject all inbound messages that weren't DKIM signed. The only time I ever had an issue was when somebody from the multinational publicly-traded company that I worked for tried to send a message to my personal inbox. They ran Exchange in the datacenter at the time (this would have been ~2017) and hadn't enabled DKIM signing. I had a friendly conversation with the sysadmin responsible for it and they had it enabled by the end of the week.
I suppose the moral of the story is that it's possible to do billions of dollars in business a year without having textbook-perfect mail infrastructure. Hell, I ran a mail server with bad MX records, a missing PTR record, and a mismatched HELO header and the world kept spinning (when I was a literal child with nobody to tell me better - I've since learned the error of my ways).