I am trying to replicate SalesForce's email-to-case logic within the application layer. However the third way SalesForce matches emails to cases doesn't seem possible. It's meant to use ClientThreadIdentifier of the EmailMessage object, which doesn't seem to exist.
This is from their documentation:
If Email-to-Case can’t find any emails with a matching In-Reply-To or References header, it checks the incoming email for an Outlook-specific header called Thread-Index. The first 22 bytes of this header uniquely identify the thread. If Email-to-Case detects a Thread-Index header on the incoming mail, it looks for matching information in the ClientThreadIdentifier field in EmailMessage records. If a match is found, the customer’s reply email is linked to the related case.
As per the title, how can this logic be replicated if SalesForce is saying its own column (as documented) doesn't exist?