Opting into a newsletter would entail adding a filter that allows messages from that address to be seen even if they do not come with a deposit.
This is actually a really good equilibrium, as it puts the newsletter whitelist into the user’s hands and if a user removes something from their whitelist the newsletter would have no way of getting around that without risking a deposit (unlike the status quo where users hitting “unsubscribe” often just confirms to spammers that their email is still a useful one to send junk to).
What if you had a contract that refunded customers based on the satisfaction of other customers?
That seems like an interesting approach too!