ICS returned from Exchange is wrong, not RFC-2445 compliant

[Peter2121]

I try to create an event on Office 365 server, using DavMail. I push the folloving ICS using Thunderbird or cadaver:

BEGIN:VCALENDAR
PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN
VERSION:2.0
BEGIN:VEVENT
CREATED:20210226T104139Z
LAST-MODIFIED:20210226T104308Z
DTSTAMP:20210226T104308Z
UID:f5dac9b4-39bc-4a2a-aa89-be1c9157f899
SUMMARY:TESTGLETBJB3
STATUS:CONFIRMED
ORGANIZER;CN=XXXXXXXXXX Pxxxx:mailto:[email protected]
ATTENDEE;RSVP=TRUE;PARTSTAT=NEEDS-ACTION;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTIC
 IPANT:mailto:[email protected]
ATTENDEE;PARTSTAT=ACCEPTED;CUTYPE=INDIVIDUAL;ROLE=REQ-PARTICIPANT:mailto:1
 [email protected]
CATEGORIES:Rendez-vous
DTSTART:20210405T100000Z
DTEND:20210405T110000Z
SEQUENCE:0
DESCRIPTION:Ne rien faire avec cette invitation
TRANSP:TRANSPARENT
END:VEVENT
END:VCALENDAR

This is an event to schedule a meeting, the organizer is [email protected], the participant is [email protected]. The ICS is created by Thunderbird.

The event is accepted by Exchange, but [email protected] lost the status of organizer. If I download the event using Thunderbird or Cadaver - I receive a wrong, corrupted ICS, not RFC-2445 compliant:

BEGIN:VCALENDAR
PRODID:Microsoft Exchange Server 2010
VERSION:2.0
BEGIN:VTIMEZONE
TZID:Europe/London
BEGIN:STANDARD
DTSTART:16010101T020000
TZOFFSETFROM:+0100
TZOFFSETTO:+0000
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=10
END:STANDARD
BEGIN:DAYLIGHT
DTSTART:16010101T010000
TZOFFSETFROM:+0000
TZOFFSETTO:+0100
RRULE:FREQ=YEARLY;INTERVAL=1;BYDAY=-1SU;BYMONTH=3
END:DAYLIGHT
END:VTIMEZONE
BEGIN:VEVENT
ATTENDEE;ROLE=REQ-PARTICIPANT;PARTSTAT=NEEDS-ACTION;RSVP=TRUE;CN="Jxxxxxxx XX
 XXXXXXX":mailto:[email protected]
DESCRIPTION;LANGUAGE=en-US:Ne rien faire avec cette invitation\n
UID:f5dac9b4-39bc-4a2a-aa89-be1c9157f899
SUMMARY;LANGUAGE=en-US:TESTGLETBJB3
DTSTART;TZID="Europe/London":20210405T110000
DTEND;TZID="Europe/London":20210405T120000
CLASS:PUBLIC
PRIORITY:5
DTSTAMP:20210226T104308Z
TRANSP:TRANSPARENT
STATUS:CONFIRMED
SEQUENCE:0
CATEGORIES:Rendez-vous
X-MICROSOFT-CDO-APPT-SEQUENCE:0
X-MICROSOFT-CDO-OWNERAPPTID:2119356428
X-MICROSOFT-CDO-BUSYSTATUS:FREE
X-MICROSOFT-CDO-INTENDEDSTATUS:BUSY
X-MICROSOFT-CDO-ALLDAYEVENT:FALSE
X-MICROSOFT-CDO-IMPORTANCE:1
X-MICROSOFT-CDO-INSTTYPE:0
X-MICROSOFT-DONOTFORWARDMEETING:FALSE
X-MICROSOFT-DISALLOW-COUNTER:FALSE
ATTENDEE;CN="Jxxxxxxx XXXXXXXXX";PARTSTAT=NEEDS-ACTION;ROLE=REQ-PARTICIPANT:m
 ailto:[email protected]
LAST-MODIFIED:20210226T142620Z
END:VEVENT
END:VCALENDAR

The first participant (organizer) is NOT present, but the second participant is mentioned twice. According to RFC-2445:

Property Name: ORGANIZER
...
Conformance: This property MUST be specified in an iCalendar object that specifies a group scheduled calendar entity.

Thunderbird tries to workaround the issue, adding the calendar's owner as an organizer and re-pushing the event to server, but during the next synchronization cycle (if the event is changed on server side) the problem comes back.

If I create a group event directly on Exchange (using Outlook or Web interface) - everything works as expected, I get the organizer correctly in ICS file.

Please, fix this major bug.

[franciscodavid]

I'm having problems with duplicate attendees similar to this one. The additional ATTENDEE property at the end is added by 'fixAttendees()' calls at

https://github.com/mguessan/davmail/blob/679fe6d92fc47ddf23bebe277c1010cc29b061a8/src/java/davmail/exchange/ews/EwsExchangeSession.java#L2184

or

https://github.com/mguessan/davmail/blob/679fe6d92fc47ddf23bebe277c1010cc29b061a8/src/java/davmail/exchange/ews/EwsExchangeSession.java#L2199

I can't find out what are they trying to fix, but if I remove the line that appends the 'fixed' attendee record at

https://github.com/mguessan/davmail/blob/679fe6d92fc47ddf23bebe277c1010cc29b061a8/src/java/davmail/exchange/ews/EwsExchangeSession.java#L2252

then, the original record is the only one listed, and the duplicate attendee goes away in my client.