DMARC is important if you want to create your own spam rules, such as "Send all e-mails from *.amazonaws.com that don't have valid DMARC to a Phishing folder".

DMARC is important if you want to create your own spam rules, such as "Send all e-mails from *.amazonaws.com that don't have valid DMARC to a Phishing folder". Sure, an attacker could use no-reply@sns.amazonaws.com.attacker.domain. But most of our users have been trained to briefly look at the From: address before clicking things, so those would not succeed often. Without DMARC, we can't even catch phishes of the real domain.