What is the cheapest transaction fee crypto coin that is still redeemable for Fiat or other crypto?

I think this concept would work if taken a step further, with a new protocol that requires encryption, but only with a key provided by a block chain that cost some nonzero amount. the email server would drop message payloads whose hash wasn't on the chain, limiting DoS attacks.

When the recipient reads the mail, it starts a process of refunding that micro payment that is a 4h cycle that can be interrupted. So if you click "spam", it blocks the refund and you keep the micro payment. Anyone sending bulk emails would go bankrupt.

Anyone using email for normal purposes would only have to buy once to have enough tokens to send a few emails. Most of the time tokens would only be used inside the system, but they would need some monetary value to do their job, so they could be pegged at say $0.10.