Apple Mail (also known as "Mail") is an email provider for customers of Apple, headquartered in California. Their postmaster includes some basic deliverability recommendations, but troubleshooting requires contacting their support team.
smtp;550 5.7.1 [CS01] Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137
smtp;550 5.7.1 Your message was rejected due to example.com’s DMARC policy. See https://support.apple.com/en-us/HT204137 for info