"Smart matching enables the e-mail router to process incoming e-mail to determine CRM relevance by comparing subject lines and party lists (user e-mail addresses in the to:, from:, cc: and bcc: lists) with e-mails already existing in CRM. The tracking token can be configured or enabled/disabled by a CRM system administrator via the System Settings dialog."
On the whole, I see this as positive, because sometimes recipients will delete the tracking token, so the email doesn't get tracked, and it is rather ugly. This does also raise some questions, mainly how smart is the smart matching? And what happens if you use the same title in different emails, and is one approach more accurate to the other?