It was discussed extensively on this list some months back, but died because two people were opposed to the idea (for diametrically opposed reasons), although many people were happy with it in principle.
For sure, I agree that using Reply-To is a pretty poor substitute for doing the job properly.
So what it needs is for someone to sit down and write an internet draft for the Mail-Followup-To header. There was a draft published some years ago by Jacob Palme which could form a basis, though it was not quite right as it stood. I would be willing to cooperate in such a venture, but I don't have the resources to edit such a draft at the moment.
regards, -- Paul Jakma paul@xxxxxxxx paul@xxxxxxxxx Key ID: 64A2FF6A Fortune: 'Martyrdom' is the only way a person can become famous without ability. -- George Bernard Shaw