I cannot 100% confirm it but "Return Path" (now Validity) definitely rings a loud bell :), and the figure is also in the ballpark -- we definitely started to send less e-mails just to be able to afford/test them.
I felt outraged at the moment because it was clearly a "pay-to-play" scheme, but ~8 years ago the number of Hotmail/Outlook addresses in my country was definitely substantial. Probably it still is.