If you don't follow the RFC then people will email you from gmail saying "I tried to send you mail from my company's mail server and it didn't work", other peop

If you don't follow the RFC then people will email you from gmail saying "I tried to send you mail from my company's mail server and it didn't work", other people will submit a github issue to your project saying they couldn't email you, and when you try to subscribe to one of djb's mailing lists you'll get a response from your mail server saying it couldn't deliver the message. That is what actually happened when I tried it.