When using Inbound Email Actions, what happens if an email is received which has no watermark or reference number?
By default, the system generates a watermark label at the bottom of each notification email to allow matching incoming email to existing records. Each watermark includes a random 20-character string that makes it unique.
https://www.servicenow.com/docs/bundle/yokohama-it-service-management/page/product/change-management/concept/change-flows.html