We use service cloud to manage our support. The auto response email when any case is created goes out from email@example.com as the from email address but the actual email address is firstname.lastname@example.org. Now when a user who receives an auto reply replies on this thread it does not get attached to the case. When trying to make the from email address same as email to case routing address it throws an error that these two cannot be same? Can someone suggest a fix to this? The reply to address is set as email@example.com.
You need to insert
Thread Id in Case Subject and Body in the email template of that auto response rule.
When the Thread ID is missing in emails sent from cases, the response email will come back without the thread ID too. Without the Thread ID, the Salesforce system can't determine which case the email is related to, so the system treats it as a new email and creates a new case.
Refer this documentation