Email-to-Case Threading: Reply Email Not Attaching Back to the Originating Case

## Expected Behavior — Working as Designed
This behavior is working as designed. Salesforce Email-to-Case Threading relies on standard SMTP email header fields (Message-ID, In-Reply-To, References) to link inbound replies to the correct Case. These are inserted into case notification emails sent from Salesforce.
## Why Threading Fails
If these header values are removed or modified before the email is received by Salesforce — for example, when the customer or their email client changes the Subject line of a reply — the threading information is lost. In this situation, Salesforce has no way to identify the originating Case and creates a new Case from the inbound email.
## Recommendation
To reduce unintended new Case creation caused by threading failures:

• Advise end-users not to modify the Subject line when replying to Salesforce case emails. Changing the subject in most email clients triggers removal of the standard email threading headers.
• If customers are reporting that their replies create new cases, review the email headers of their reply messages to confirm whether the In-Reply-To and References fields are present. See the Additional Resources below for guidance on finding email headers.