Interesting take. The refund model is smart — turns it into a deposit rather than a fee. We've thought about configurable pricing where each user sets their own rate.
The tension: would kill nearly all spam but also kills legitimate cold outreach (job offers, old friends reconnecting, etc). 100 sats (~8¢) is low enough that anyone with something real to say won't hesitate, but high enough that mass-sending becomes expensive fast.
User-configurable pricing might be the answer. Set your own price for your own attention.