Message Tracking (MT) is a tool that is part the Exchange System Manager (ESM). It reports what a given Exchange server does with particular messages. Note that tracking ends when the message leaves that Exchange server. If the message goes to another Exchange server, you can consult MT on that other server for more information.
MT can be used to determine if a message was delivered to a mailbox, or to another system. If the tracking ends without the message being delivered, you will see what Exchange was doing last. That can give clues as to what the underlying issue is.
MT can be enabled or disabled. Open the properties of the Exchange server. On the General tab there is a checkbox to Enable Message Tracking. There are also settings for log location and retention.