[CryptoPunk]

Fiat means giving the payment processor significant leverage over the paid email service.

Fiat also means the email service couldn't be global, because Trusted-Third-Party-based payment processors can't operate globally owing to the global regulatory patchwork.

Contrast that with Bitcoin(Cash)-based international remittance, or Ethereum-based token sales, which are accessible to people in every country in the world.

Email is an open and global protocol, so cryptocurrency is a good fit for it.