Not that I can share, just have had this conversation with product and run experiments at a couple places. And the conversation was me saying won’t increasing the top of the funnel increase the bottom since that seems most logical, and being wrong. The ymmv at the end was a weak way of saying this is anecdata so you’re right to question it.

I don’t think not wanting to receive spam is the right comparison. It’s more didn’t see value in the program by the website, therefore won’t see the value running it. They’ll try it for five minutes then forget about it. Assuming the emails are causing more engagement, whatever is getting them to use the program from emails should be in the website. To me this is the problem that is worth solving.