Email correlation is a process where CRM links tracked emails with records in the database based on information in the email.
In earlier versions, Smart Matching and Tracking Tokens were used to correlate emails.
Microsoft Dynamics CRM 2013 introduces a new method to match emails which uses information in email headers. It is claimed that it is more accurate than Smart Matching. However, the method (which is labelled Correlation in the Email tab in System Settings) only applies to Server-Side Synchronization and the CRM Client for Outlook. The Email Router does not use this method.