Best practice for email correlation between ServiceNow CSM and customer's ticketing system

sn-ushi — Wed, 26 Aug 2026 12:10:43 GMT

Hi all,

we have a CSM email integration scenario where both ServiceNow and the customer use their own ticketing systems.

The customer sends emails from their ticketing system to our ServiceNow CSM mailbox. Their external ticket number is part of the subject, for example:

Ticket: EXT-123456 - "open tickets"

ServiceNow creates a Case, e.g. CS0012345, and sends an automatic notification back to the customer's service desk.

The ServiceNow notification subject is currently something like:

CS0012345 - A new case has been opened

ServiceNow also adds its normal Ref:MSG... watermark to the email.

The problem is that the customer's ticketing system requires its own ticket number (EXT-123456) in the subject to associate the incoming email with the existing ticket. If the external ticket number is no longer present, the customer system may create a new ticket, potentially resulting in an email/ticket ping-pong.

On the ServiceNow Case, correlation_id and correlation_display are currently empty.

What is the recommended ServiceNow approach for this kind of email-to-email ticket system communication?

In particular:

Is there an OOTB/recommended mechanism to retain an external ticket reference from an inbound email and reuse it in outbound notification subjects?
Should the external ticket number be stored in correlation_id, or is that field intended for a different correlation scenario?
Would you recommend storing the customer's ticket number in a dedicated Case field and using it in the notification subject?
How should this work together with ServiceNow's existing Ref:MSG... watermark?
Is there a standard approach to prevent autoresponse loops when both ServiceNow and the external ticketing system generate automatic notifications?
Ideally, this behavior should be configurable only for customers/service desk mailboxes that require external ticket correlation.

question Best practice for email correlation between ServiceNow CSM and customer's ticketing system in Community Central forum

Best practice for email correlation between ServiceNow CSM and customer's ticketing system