This mail cannot be forwarded because it was detected as spam

Hello. A customer set up a forwarder for his account to icloud (another customer uses gmail, same behaviour). He is receiving emails fine. But, I'm seeing lots of emails in the server's email queue being sent back to the original sender, with the message: From: Mail Delivery System To: mycustomeremail@domainhosted.com This message was created automatically by mail delivery software. A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed: mycustomeremail@icloud.com (ultimately generated from mycustomeremail@domainhosted.com) This mail cannot be forwarded because it was detected as spam. So, the local account mycustomeremail@domainhosted.com forwards to mycustomeremail@icloud.com How can I prevent this messages to be sent back to sender ?