“Emails are complicated. If you haven’t come to that conclusion by this point, I’d like to talk to you. It’s an old system that’s held together by a bazillion standards that have been written over decades. People do unexpected things with emails. There are intricate systems that were set up in the 90s that still run, unobstructed, on top of email.”
https://simonandrews.ca/articles/how-to-set-up-spf-dkim-dmarc