@GlideFather Thank you for the quick response

Based on my understanding, the email will be classified as reply before reaching to inbound action. Then vendor's watermark convention might not be useful. Could you suggest on this

1.) Email arrives → engine scans for watermark
2.) Watermark found → matched against sys_email_watermark table
3.) Email classified as Reply → routed to existing record
4.) New inbound action rules → never evaluated