[Air-l] First use of opt-in mailing?

A historical question has come up on another list: when did companies first start offering webpage visitors "opt-in" options for emailing, that is, the boxes or other mechanisms that allowed people to opt in for mailing from other companies?

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Who was the first to use those little opt-in checkboxes online? I know NetCreations started in 1996 or so, but I seem to remember doing stuff earlier. Does anyone have any firm dates? I have tried Google, WayBack, etc, and the earliest firm date I can find is a general 1996.

Thanks for any leads, Gina