This code indicates a looping condition. This issue may occur if one of the recipient policies includes a local domain that ...

{4}: This code indicates a looping condition. This issue may occur if one of the recipient policies includes a local domain that matches the fully-qualified domain name (FQDN) of an Exchange server in the organization. When the categorizer processes mail that is destined for a domain that matches the fully-qualified domain name (FQDN) of an Exchange server, a non-delivery report with this code is generated.