A report is rejected instead of creating a case when its domain doesn't belong to any zone your account monitors. Every account has a single default zone for inbound reports, covering any origin (manual entry, API, or email). Domains outside that zone, and outside any other zone you monitor, are rejected outright rather than being filed under an unrelated zone.
If a case for the domain already exists, a new report is added to that case instead of creating a new one; it isn't rejected. Search Cases for the domain if you're not sure.
If a domain should be covered going forward, add it to a zone your account monitors. See How do I add domains to be monitored?
If you believe a domain is already covered by one of your zones, and a report for it should not have been rejected, contact Support. Include the domain name and approximately when the report was submitted so the team can check it. You can email support@iq.global, or use the message icon in the bottom right corner of Abuse Manager.