Why aren't my emails getting delivered? 554 5.7.1 [CS02] Message rejected due to local policy.

I'm trying to send emails to clients but whenever I send to @icloud.com emails I get this error message back: 554 5.7.1 [CS02] Message rejected due to local policy. Please visit Postmaster information for iCloud Mail - Apple Support

I've tried the Apple support site but nothing helps.

As I understand it, our emails have been reported as spam at some point? This customer wrote me for a refund, so I would like to contact them but they're not using the same email in our database, so I can't even find their phone number.

Hope someone can help me. Thank you!