By default, Outlook deletes a meeting request from your Inbox once you respond to the meeting request. While Outlook does add the meeting and its details to your Calendar, it is much easier to also have these details in your Inbox for easy reference. To keep your meeting requests from being deleted from your Inbox, follow these steps:
1. Click Tools and select Options.
2. On the Preferences tab, click the E-mail Options button.
3. Click the Advanced E-mail Options button.
4. In the When sending a message section, uncheck the Delete meeting request from Inbox when responding checkbox.
5. Click OK.
6. Click OK to close E-mail Options and again to close Options.