Whenever a meeting request arrives in your Inbox, the Outlook sniffer processes the item and places a "time blocker" in your calendar as a tentative appointment.

The important difference between the sniffer's action and what happens when you accept an appointment as tentative is that the originator of the meeting doesn't receive any kind of response when the sniffer processes the appointment. Also, the sniffer doesn't process appointments automatically when they arrive: You either have to open the item or view it in the preview pane for it to be processed.

The sniffer was implemented by Microsoft in Outlook 2000 and has been processing Outlook appointment requests ever since; it can't be turned off. The only workaround to prevent this behavior is to move appointment requests out of your Inbox before the sniffer can process them. You can do this by using the Rules Wizard. Follow these steps to create the rule:

  1. Create a folder where you want all new appointment requests to go; make sure the Reading pane is turned off on this folder.
  2. Click on your inbox. From your home tab, go to Rules > Manage Rules > New Rule.
  3. Select Check messages when they arrive, then click Next.
  4. On the Select conditions page, select which is a meeting invitation or update.
  5. On the next page, select move it to the specific folder and define the new folder you created as the target.
  6. Click Finish to create the rule.

With this rule, any incoming appointment request will be automatically moved to the folder you chose. You have to access this folder to respond to the request. Appointments will no longer automatically appear in your calendar. However, the sniffer works in all your folders, not only the Inbox. So, when you look at requests in the new folder, you should be prepared to respond immediately if you don’t want to see the tentative meeting on your calendar.

Source: Windows IT Pro - How the Outlook Sniffer Processes Your Appointments. (2008). Retrieved 4/9/13
http://windowsitpro.com/exchange-server/how-outlook-sniffer-processes-your-appointments