title: Outlook doesn't send answer to event invitation via iCal
body: <p>I am trying to send an invitation to an event in form of an iCal email attachment. I'm testing it with 3 clients: Zimbra, Gmail and Outlook. All three interpret the attachment correctly: They show the respective RSVP buttons (Accept/Tentative/Decline). However, after pressing the button, only Zimbra sends an email with an iCal file containing the answer. Gmail and Outlook don't.</p>
  
  <p>Is this a normal behavior, or am I doing something wrong? I would expect the clients to send an email with an answer. Saving the data only to a local calendar doesn't make much sense to me.</p>
  
  <p>Example of the iCal I am sending:</p>
  
  <pre><code>BEGIN:VCALENDAR
  VERSION:2.0
  CALSCALE:GREGORIAN
  PRODID:-//Calendar APP
  METHOD:REQUEST
  BEGIN:VEVENT
  DTSTAMP:20180424T084322Z
  DTSTART:20180424T000000
  DTEND:20180424T235959
  SUMMARY:e107
  TZID:Europe/Vienna
  LOCATION:Besprechungsraum 1
  SEQUENCE:0
  ORGANIZER:mailto:organizer@mail.com
  UID:20180424T084322Z-confId=795:timeTableId=18997@fe80:0:0:0:e73:2050:cc3
   d:6035%utun0
  ATTENDEE;ROLE=REQ-PARTICIPANT;CN=;PARTSTAT=NEEDS_ACTION;RSVP=TRUE:mailto:
   user@mail.com
  ATTENDEE;ROLE=REQ-PARTICIPANT;CN=;PARTSTAT=NEEDS_ACTION;RSVP=TRUE:mailto:
   another_user@mail.com
  END:VEVENT
  END:VCALENDAR
  </code></pre>
  
  <p>Thank you in advance.</p>