At a glance, you can quickly see your agenda in daily, weekly, or monthly views.With Outlook, you won’t have any excuses for missing a lunch date or forgetting a meeting.You can also see who has not yet Tracking Status tab, you'll see the responses shown as a table, but unfortunately it's not possible to sort or filter that list - so quickly picking out everyone who hasn't responded from a long list of invited people isn't so easy.Use these instructions to sort/filter the responses.
In other words, it will appear as though the attendee has not responded even though the meeting has been accepted or declined.
How can I see if my message was read or delivered successfully within the sent item?
Message tracking via the Internet is a bit tricky as there are quite a few points at which it could get broken.
Make sure that the box for "Automatically procss meeting requests and responses..." is checked.
If it's not then Outlook won't try to do anything with those responses.