The old Kayako (Kayako Classic) checks the subject and the body of the email for ticket IDs to determine where a customer's reply to the ticket should go. You may want to know how the new Kayako handles this process.
This article explains how the new Kayako (TNK) routes your customers' emails to certain tickets.
The new Kayako processes the information that is present in headers of emails to route them appropriately. For every email which comes into Kayako, it looks at the following header information:
For example, the original message ID from the email that started the conversation:
If an email contains any of these headers, it will be processed and linked to the existing conversation that has the same header information. If the information is unique (not present in any of the tickets), Kayako will create a new conversation.