If DMARC were mandatory for all email, we'd still see plenty of spam. All snowshoe spam, for example, uses DMARC in order to look like a legitimate marketer and get the free passes that
All DMARC does is prevent spoofing of the From header's domain. You can still set up your own "marketing" domain and spew spam. You can still register bankofamerica-customersupport.com or create an account for "firstname.lastname@example.org" or hack into "email@example.com" and change the friendly-from to "Bank of America Customer Support" and not worry about the email address since software like Apple iOS's Mail app will only show the friendly-from. Solving that kind of forgery is much harder. Trust me, it's part of my job.
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