smtp;554 5.7.1 [CS01] Message rejected due to local policy. Please visit https://support.apple.com/en-us/HT204137
Apple Mail is blocking the email because of an email filter policy on their end. If this is happening for a specific recipient, ask them to save the From address for the message as an iCloud contact, specifically marking them as a "VIP" there. This VIP status should at least temporarily override the block. If you're following Apple's suggested best practices, you can also contact Apple's postmaster team with mail logs for delivery support.