By default, when using Inbound actions, what happens if an email is received which has an Incident watermark?
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-platform-administration/page/administer/notification/concept/c_WorkingWithWatermarks.html