Anyone of the following will prevent email notifications from being sent:

1. You have added a Delegate and selected 'Send meeting requests and responses only to my delegates, not to me'.
2. You have your calendar set to auto-accept meeting requests.
3. You set 'Author' permissions on your calendar. This allows a person to open your calendar by choosing 'Open a Shared Calendar' and schedule a meeting directly on your calendar. Therefore, no email notification is sent.