I think that this error is triggered by Apple, not Intercom. Seems like Apple blocks messages on their iMessage network if they violate local policy (content, links, or attachments).

That error is actually coming from Apple’s mail servers, not Intercom. It usually means the recipient’s mail system rejected the message based on their own security or spam policies. The fix will likely need to be on your side — things like checking your SPF, DKIM, and DMARC records and making sure your sending domain is properly authenticated. Intercom just passes the message through, but Apple is the one rejecting it.