The famous Ben Hosk wrote up a great little post about the e-mail tracking token. I like to call it half of the locket or an automation puzzle piece. It clicks a responding e-mail into the e-mail that is being responded to. Offering incredibly reliable threading. On the other hand it is TOTALLY Optional and Dynamics CRM is sophisticated enough to have even more embedded algoritums of success.
If you would like to know more about the e-mail tracking token, take a peek at Ben's writeup which you can find here.