How It Actually Works Under the Hood

Eight posts in, and I haven’t talked about how Rythm actually works under the hood.

If you’ve been following along, you know the concept: known senders reach your inbox, unknown senders wait. If an unknown sender wants to skip the line, they pay a small cover charge. The money goes to you. We call this economic email filtering, and the deterministic approach behind it is what makes it work.

But how does a web app process payments between two strangers without holding anyone’s money? Without running financial infrastructure? Without a money transmission license?

This is the engineering problem that defined how we built Rythm. And the answer is the reason I believe open protocols are going to fundamentally change how software gets built.

The payment flow

Here’s what actually happens when someone pays to reach your inbox.

The sender visits a paywall page. Rythm requests a quote from a public Cashu mint. The mint generates a Lightning invoice. The sender pays that invoice with whatever Lightning wallet they already use. Strike, Cash App, Phoenix, it doesn’t matter.

Once the mint confirms payment, it issues the sender a Cashu token. Think of it like a stamped receipt. A small piece of cryptographic proof that says: this person paid.

The sender pastes that token into their email and hits send. The token travels with the message, like a dollar bill tucked inside an envelope.

When the email arrives, Rythm does three things. It detects the token in the message body. It checks with the mint to confirm the token is real and unspent. And it redeems the token to your Lightning address, which generates a payment directly to your wallet.

Then Rythm discards the token and the message content from memory. Nothing stored. We go deeper into the cryptographic and architectural details in our non-custodial architecture post.

The entire flow takes seconds. And at no point does Rythm hold, custody, or transmit anyone’s money. Funds move from the sender to the mint to your wallet. Rythm is just the automation layer that validates the token and triggers the redemption. The same actions you could take manually if you opened the email, copied the token, and redeemed it yourself.

Why we built it this way

We didn’t build it this way because we’re ideological about it. We built it this way because it was the only architecture that let us stay focused on what we’re actually good at: building useful software.

The alternative was spending millions on money transmission licenses, legal infrastructure, and regulatory compliance just to process a four-cent payment on someone’s inbox. That cost would have killed the product before it started, or been passed directly to you.

Open protocols gave us a different path. Cashu handles the payment proof. Lightning handles the settlement. LNURL gives users a simple, familiar way to connect their wallet, just a Lightning address like [email protected]. We didn’t have to build any of that infrastructure. We just had to build on top of it.

The unlock

This is what I think most people outside the Bitcoin ecosystem haven’t fully grasped yet. Open value protocols do for payments what TCP/IP did for communication. They let builders focus on solving real problems instead of rebuilding the plumbing from scratch and asking for permission to turn it on.

What’s not perfect

Is the payment flow perfect today? No. Pasting a token into an email body has friction. We know that. We’re actively working on reducing it. As new tools, interoperability solutions, and open-source software mature, this flow will get smoother. The architecture is built to evolve.

But the principle won’t change. Rythm will always remain non-custodial. We will never hold your money. We will never require a centralized account. The value exchange happens on open rails between the sender and you, and Rythm just makes sure the math checks out. We wrote about this privacy commitment in all muscle, no curiosity.

This is what building on a complete foundation looks like. Not a fintech product. A useful tool that happens to run on open protocols designed for exactly this kind of problem.