Spam exists because email can be sent from any domain on the internet by default without requiring any validation. The moment that enforced DMARC with p=reject

Spam exists because email can be sent from any domain on the internet by default without requiring any validation. The moment that enforced DMARC with p=reject is mandatory a lot of problems will go away because you will be required to "turn on" email for your domain with SPF and DKIM. In the mean time, every domain that has ever been registered is subject to being used for spam.