I am working with iCloud, iPhone and Macbook Pro on Yosemite.
I receive ICS calendar invites via mail from a user using Outlook. When I click on the ICS file in the mail, it opens and I can add it to my calendar without a problem. It also syncs nicely on all my devices.
However, the sender of the invite does not get a confirmation, in the meeting properties I'm shown as "did not reply". When I open the invite in my calendar, I have no option to accept or send a notification.
In short: ICS invites are added to my calendar but the organizer does not receive a notification.
How can I send an accept notification?