I am using the Lightning calendar to view a Google calendar which imports another calendar as Free/Busy calendar. This means that appointments contain only start time, end time and the text "Tentative" and "Busy". I now received an e-mail with an appointment invitation and Thunderbird block access to this invitation and the details in the mail because the appointment is contained in this free/busy calendar (without any details of course). '''How can I access the original invitation in Thunderbird to see the appointment details? Alternatively I would like to disable this whole system that blocks access to invitations because "This message contains an event that has already been processed." ''' Other answers on similar questions here in the forum suggest to disable Lightning which is not working for me because I am using Lighning with multiple calendars. I just don't want Thunderbird to block access to an invitation mail. Can somebody help me how to disable this annoying behavior of Thunderbird?