I'm trying and failing to get .ics files (that have been sent as an email attachments) to be automatically added to the user's calendar. This Apple discussion suggests this has been stopped, but I'm almost certain that I've been sent .ics files since then that have been automatically added to my calendar.
I don't think the problem is with the .ics files; I took the .ics file that I was emailed and which I'm sure was automatically added to my calendar, and tried sending it to myself, but to no avail (I can click on it and it then gets added an works perfectly). I presume there's a problem with the email headers, which may be mismatching sources/authentication. I tried this SO answer but it didn't work.