My question is similar to this question, but I would like to filter calendar notifications that came from applications other than Google Calendar. I have been successfully filtering Google Calendar notifications using the filter
Has the words: *.ics ("Invitation:" OR "Accepted" OR "Declined"), but have not found a way to filter calendar notifications from Outlook users.
Edit: Here's a screenshot of an email invitation sent from MS Outlook. Unfortunately, since it's a work account, I can't include the message source. The only reference the source contains to "calendar" is
Content-Type: text/calendar;. It appears as though Gmail is automatically parsing some information and creating the calendar box shown above the text of the email.