As of version 9.18 of Email to Case Premium, the order of operations for matching inbound emails to their associated open Cases is as follows:

1. In-Reply-To email header 
2. References in email header 
3. Lightning Thread Token (if enabled) 
4. Legacy E2CP Thread ID

Sometimes, Organizations may question how certain email messages were threaded to a particular support case, and this knowledge article aims to explain how to identify which threading behavior is happening in those scenarios.

Scenario 1: A person uses the same email message and simply updates the recipient and the content of the email, instead of creating a new email every time. 

Scenario 2: If a business process requires sending an email to create a support case to log documents, action items, deliverables, etc., someone may send one email to the target support address, then do a reply all to the sent email, simply updating it with new recipients.

Suppose the Salesforce Organization processing those emails has Message-ID-based threading in place, including having the "Enable Message-ID Threading" setting in Email to Case Premium enabled. In that case, any replies to an email that creates a case will be threaded to the original case.

To confirm that this is the case, look at the email headers in Salesforce (if you captured them) or open the original email message in your email client and review the message details. Look for "References:"and "In-Reply-To:". If these values match the email message that originated the support case, that is why they are being threaded to that same case. 

The headers in the email will all be the same, and the Message-ID Threading will connect the email to the first email's support case because of the matching headers headers.

To ensure the appropriate threading occurs, always initiate a new email message and do not update only certain aspects of it prior to sending out multiple messages to different audiences.