Retrieves a calendar event object.

• For Outlook, the attendee list of an invitation the connected account received is the one the organizer sent, and their responses are not refreshed on it. Only the response of the connected account is kept up to date. Read the event from the calendar of its organizer to get the responses of the other attendees.
• For Outlook, an account invited through one of its aliases is not identifiable in the attendee list: Microsoft does not support an alias taking part in a calendar, so the invitation keeps the alias address while the account answers with its primary one.