Apple / SMTP error code 550
smtp;550 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 you're unsure of the source of this issue, and following Apple's suggested best practices, you can contact Apple's postmaster team with mail logs for delivery support.
Documentation
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