I sometimes receive emails with .ics file attachments. Mail.app automatically adds these events to my iCloud Calendar as soon as I preview the message.
How can I disable this behaviour and only add the event if I click on the attachment manually?
In the Mail.app go to Preferences->general. There is an option "Add invitations to ICal" which is Automatic or None. You need to choose None.
When you have a mail with date and time in Mail.App will highlight that and clicking on that will add an event to iCal.