Disposition-Notification-To: Luke-Jr <luke-jr@utopios.org> header requests notification from reader when message is viewed. To make things worse - in many clients you don't even need to open the message - just preview is enough. Besides being plainly annoying, this partially exposes subscriber base to sender. Not very good thing, IMHO. I think we need to strip this flag from ML messages, but also have some generic policy for things like that.
We strip the Disposition-Notification-To header in all lists now. Thanks for the info