Hi, strictly speaking this isn’t about authentication, but I’m not sure where else it would fit. The “Joe (@Joe) is now following you on Twitter!” emails that come to my account used to include special “X-Twittersenderscreenname” and “X-Twittersenderid” headers, showing the user ID of the person following. This header was inside the email headers, along with all the usual email headers.
They’re gone now, and they have apparently been gone for a long time. Why? These were wonderful courtesy values to have tucked in the header; without them, I have to parse the email and regex out the person’s name, and risk my regex breaking if Twitter updates the email format. (And please don’t tell me it was for some anti-spam reason-- anyone can regex the info-- it was just giving me a much easier way to get what I needed.)