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we don't have any data showing how much of these warnings convincing people for not to print it. If they are protecting more papers then the extended ones it means that it's still working. Mail platforms should capture and hide them while printing.
Each email is one line longer, meaning that a whole extra pages is printed some times. Versus the change in peoples behaviour which there doesn't seem to be any evidence for.
if you write it often enough in the footer the email gets long enough (in extreme cases ~100 pages) so the person stops printing your mails
The intention may not be to actually have any impact on the environment, but to communicate that the sender 'cares', which it may be effective at.
If in fact it is counter productive, the sender is trading others perception of themselves for the environment.