Email Inbound action updated record without watermark and without number in the subject - Support and Troubleshooting

Email Inbound action updated record without watermark and without number in the subject Issue  How is it possible that a reply to the original Email that was first sent to an instance can end up updating the target case although that original Email does not contain anywhere a record number, nor even a watermark ?

Email A sent out to an instance Email A received and Inbound Action processes Email and creates a new record Email sender, opens up original Email in Outlook and sends a reply to that same Email.

Release All

Cause Expected behaviour that Email should open a duplicate record

Resolution When a received email does not have watermark and record number, ServiceNow checks the email in-reply-to header.

If the header contains a value that matches a previously received email message that resulted in a record creation, then the email is classified as a Reply to that record and the record is updated