title: Outlook fails to recognize custom UID in .ics file, creates new event (and new id)
body: <p>What I am trying to do:<br>
  I have a need to send my clients <code>.ics</code> files that will populate their Outlook calendars as soon as they download and double click (and choose to populate their Outlook calendar).<br></p>
  <p>When I am testing locally, the iCalendar code below works fine if I set Outlook to work <code>offline</code> - a UID is generated <code>once</code> for each event, I can use the same file again and again and a new event will <code>not</code> be generated - as the UID is recognized by Outlook as already existing. (so I can rely on the UID to update, and cancel as well)<br></p>
  <p>The issue is that when I work with an <code>online</code> Outlook, a random UID (112 characters long) is generated every time I use the file to populate the calendar - even for the same events - so a duplicate event is generated <code>every time!</code><br><br>
  Example for an .ics file: <br></p>
  <pre><code>BEGIN:VCALENDAR
  
  VERSION:2.0
  METHOD:PUBLISH
  PRODID:-//PYVOBJECT//NONSGML Version 1//EN
  
  BEGIN:VEVENT
  UID:12345
  DTSTART:20190715T020222Z
  DTEND:20190715T030222Z
  DESCRIPTION:Some fake summary - first event
  DTSTAMP:20190715T010222Z
  SEQUENCE:0
  SUMMARY:this is the first event summary
  END:VEVENT
  
  BEGIN:VEVENT
  UID:67890
  DTSTART:20190715T040222Z
  DTEND:20190715T050222Z
  DESCRIPTION:Some fake summary - second event
  DTSTAMP:20190715T010222Z
  SEQUENCE:0
  SUMMARY:this is the second event summary
  END:VEVENT
  
  END:VCALENDAR
  </code></pre>
  <p>If I generate an .ics file for the first event that was generated when Outlook is <code>online</code> (dragging and dropping out of the calendar) I see that Outlook has added a bunch of new fields:</p>
  <pre><code>BEGIN:VCALENDAR
  VERSION:2.0
  PRODID:-//Microsoft Corporation//Outlook for Mac MIMEDIR//EN
  METHOD:PUBLISH
  BEGIN:VTIMEZONE
  TZID:Pacific Time (US &amp; Canada)
  X-ENTOURAGE-CFTIMEZONE:US/Pacific
  X-ENTOURAGE-TZID:1
  BEGIN:STANDARD
  RRULE:FREQ=YEARLY;INTERVAL=1;BYSECOND=0;BYMINUTE=0;BYHOUR=2;BYDAY=1SU;BYMONTH=11
  TZOFFSETFROM:-0700
  TZOFFSETTO:-0800
  DTSTART:20071104T020000
  END:STANDARD
  BEGIN:DAYLIGHT
  RRULE:FREQ=YEARLY;INTERVAL=1;BYSECOND=0;BYMINUTE=0;BYHOUR=2;BYDAY=2SU;BYMONTH=3
  TZOFFSETFROM:-0800
  TZOFFSETTO:-0700
  DTSTART:20070311T020000
  END:DAYLIGHT
  END:VTIMEZONE
  
  BEGIN:VEVENT
  UID:&lt;a NEW 112 character id - NOT the one I added myself&gt;
  X-ENTOURAGE_UUID:81760164-FB2C-4421-B7AC-2E1791265F0A
  X-MICROSOFT-EXCHANGE-ID:&lt;base64, 140 character id&gt;
  X-MICROSOFT-EXCHANGE-CHANGEKEY:&lt;base64, 32 character id&gt;
  DTSTAMP:20190715T023949Z
  DTSTART;TZID=&quot;Pacific Time (US &amp; Canada)&quot;:20190714T190200
  DTEND;TZID=&quot;Pacific Time (US &amp; Canada)&quot;:20190714T200200
  LAST-MODIFIED:20190715T023949Z
  SUMMARY:this is the first event summary
  DESCRIPTION:Some fake summary - first event
  ORGANIZER:MAILTO:&lt;the real account owner email - my email&gt;
  SEQUENCE:0
  X-MICROSOFT-CDO-BUSYSTATUS:BUSY
  X-MICROSOFT-CDO-ALLDAYEVENT:FALSE
  X-MICROSOFT-DISALLOW-COUNTER:TRUE
  X-MICROSOFT-DONOTFORWARDMEETING:FALSE
  X-MICROSOFT-CDO-INSTTYPE:0
  END:VEVENT
  
  END:VCALENDAR
  </code></pre>
  <p>What I tried:<br></p>
  <ol>
  <li>Modified: <code>METHOD:REQUEST</code><br></li>
  <li>Added <code>X-WR-RELCALID:some_unique_id</code> <a href="https://stackoverflow.com/questions/45453/icalendar-and-event-updates-not-working-in-outlookhttps://">from this post</a><br></li>
  <li>Sequence does not seem to help - it is only there to sort out if 2 UIDs are the same within the same file<br></li>
  <li>I added <code>ORGANIZER;CN=Fable mail:MAILTO:fable.mail@example.com</code> to each event - it has the same issue as the UID, and is disregarded (my email is added as the organizer)<br></li>
  <li>I added <code>ATTENDEE:MAILTO:somemail@email.com</code> as suggested in <a href="https://stackoverflow.com/questions/46509045/google-calendar-ignoring-ics-uid-when-using-add-to-calendar-link">this post</a>, but it does not seem to make any difference<br></li>
  </ol>
  <p>Is it because I am not using the <a href="https://www.rfc-editor.org/rfc/rfc5545#section-3.8.4.7" rel="nofollow noreferrer">recommended standard</a> for the UID? It seems like a suggestion to prevent collisions, nothing more.<br></p>
  <p>Thank you!</p>