Calendar invitations can really clutter up your calendar! This article describes how to keep Outlook from accepting all invitations as "Tentative" and how to keep them from appearing on your calendar before you have accepted them.
By default, Outlook 2010 and Outlook 2013 will automatically
process your meeting requests so these time-slots are reserved on your
Calendar as Tentative. This is to prevent others from booking the same time-slot before you have processed an earlier sent meeting request.
The automatic processing of meeting requests can be controlled via your Tracking options.
While disabling this option will prevent time-slots from being booked automatically as Tentative, it also requires you to manually process any responses that you get to your invitations and polls.
In order to process the received responses, it is sufficient to view the response in the Reading Pane or to open the response via a double click.
Tracking options allow you to automatically process certain responses and requests.
Note 1: Even though a meeting request will no longer be automatically added as Tentative to your Calendar upon receiving, it will still be added as Tentative as soon as you select the meeting request in your message list.
Note 2: Disabling this option can also come in handy when you receive a lot of spam messages in the form of meeting requests.