Why does Mail.app automatically add iCal events from invitations I receive? 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?
There are web app preferences to manage this in newer macOS.
In the Mail.app go to Preferences-&gt;general. There is an option &quot;Add invitations to ICal&quot; 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.