icloud rejects emails from our server
Our order processing system sends purchase confirmations when a transaction is completed. We use AWS for the actual sending. Email sent to any icloud.com account is rejected. We aren't having trouble with any other email provider (Google, Microsoft, Yahoo, and many other smaller ones). When I check an email that I send to my own gmail account, it has passed all the DMARC and other validation checks.
For now, we have stopped people from specifying icloud email addresses in our system. Hopefully this issue will be corrected and icloud users on our system won't continue to be inconvenienced.