Microsoft sez:
"Soon after a meeting request arrives in your Inbox, a piece of Outlook code
— nicknamed the "sniffer" — automatically adds the meeting to your calendar
and marks it as tentative. This is a fail-safe to keep you from missing the
meeting in case you don't see the request in your Inbox. However, the sniffer
doesn't reply to the meeting organizer. You still need to do that by
accepting, accepting as tentative, or declining the request."
