While working on BKO458524 I sent a counter-proposal from KMail to an invitation I created in Outlook365 (the web version of Outlook on outlook.office.com).
Outlook displays the attached .ics file as a regular attachment with title "not supported calendar message" and refuses to even dispaly it, although I know for sure that Outlook does support counter-proposals to invitations - so either we are sending completely invalid counter-proposals, or we are doing something that Outlook doesn't like.
- Check what does a counter-proposal sent from Outlook look like, which should help identifying what does Outlook expect a counter-proposal to look like
- Find out whether we can transparently support this in the iTIP plugin in KMail
- Implement it :)