meeting invitations to Exchange resources and Outlook users

Dear list,

After trying numerous things I just cannot send a meeting invitation from 
KOrganizer via KMail in a way that Exchange/Outlook 2010 will understand.

* I have tried all checkboxes in KMail's "Misc - Invitations" options in 
various combinations
* I have tried forcing it to be sent as utf-8 (encountering this weird bug by 
the way: https://bugs.kde.org/show_bug.cgi?id=147924

No results however:
* people receiving the invitation always get a mail with a .ics attachment. 
Outlook does not detect it as an invitation (and shows no UI for 
accept/decline etc.)
* when adding a resource (meeting room) to the attendees, it never responds 
(i.o.w. Exchange does not recognize the invitation and silently deletes the 
mail)

I can live with attendees manually having to open the .ics. However, it's 
impossible to book a meeting room from Kontact.

Meanwhile Thunderbird + Lightning does manage to do it somehow. I see a number 
of differences in the sent e-mail, which I don't know how to emulate using 
KMail:

* multipart/alternative message body containing 2 parts: the description 
(text/plain), and the vcalendar text (text/calendar). This is the first part of 
the whole multipart/mixed mail (which is hence containing this 
multipart/alternative first, and next the .ics file attachment as the second 
part (application/ics)).
KMail is multipart/mixed only, with a text/plain message body and 
text/calendar .ics attachment.

* Content-transfer-encoding: 8BIT for all parts (I can get KMail to use the 
utf-8 charset, but encoding is always quoted-printable.

* KMail uses "cal.ics" as attachment filename, Lightning uses "invite.ics"

I found this old post interesting, apparently some of the "Misc - Invitations" 
checkboxes were aimed at Exchange 5.5 (1997):
http://lists.kde.org/?l=kde-pim&m=110379611900121

I'm using KDEPIM version 4.12.2. Anything left to try?

Best regards,

Pascal
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