Having to tell an ordinary user that their domain didn't configure DKIM properly so sign up didn't work and they now have to go through an alternate flow sounds

Having to tell an ordinary user that their domain didn't configure DKIM properly so sign up didn't work and they now have to go through an alternate flow sounds like a nightmare. The article does not mention what to do with the many users who don't already have the mentioned security mechanisms, just that they are "commonly deployed".