Use custom thread IDs with Email-to-Case

Salesforce does not support the use of compound custom thread IDs in Email-to-Case. Custom thread IDs that are composed from formulas or custom fields cannot be accommodated when Salesforce makes application-side changes to the thread ID processing logic.

Why Custom Thread IDs Cause Problems

The Email-to-Case thread ID is processed by Salesforce's routing engine to match inbound emails to the correct case. When a custom thread ID is used, the routing engine may fail to match the email if:

• The thread ID format does not match the expected pattern used by the routing engine.
• Salesforce updates the thread ID generation logic in a platform release, invalidating existing custom IDs.
• The custom formula produces different output depending on field values at the time of case creation vs. the time of email reply.

Recommended Approach

Use the out-of-the-box Salesforce-generated thread ID. The system thread ID is reliable, maintained across platform releases, and fully supported by Salesforce Support.
If your organization requires a custom identifier in emails for tracking purposes, add it as a separate custom field or in the email body, but do not replace the system thread ID.
If you are experiencing thread ID matching failures after a Salesforce release, contact Salesforce Support and reference this article.