The Sender Policy Framework (SPF)
www.netmeister.org/blog/spf.html
What Jamie wrote
I've had to figure out enough of SPF in order to configure mail for thingelstad.com, and when you start doing a newsletter the mail settings for your domain get much more important to avoid spam folders. This dig into SPF was a fun rabbit hole. I actually LOL'd for real when I got to the section on macros. I had no idea SPF supported macros! 🤣