The Rythm glossary — the terms, defined.

Rythm is a dual-layer deterministic email filter for Gmail and Outlook, powered by the Cashu ecash protocol over the Lightning Network. If any of those words are new, this page defines them all in plain English.

Email paywall

A deterministic filter that requires unknown senders to pay a small cover charge to reach the inbox. Rythm is an email paywall for Gmail and Outlook.

Dual-layer deterministic filter

Rythm's filtering architecture: Layer 1 is identity (the guest list); Layer 2 is cost (the cover charge). Both are user-controlled. Binary logic, no guessing.

Guest list

Your list of approved senders. Auto-built from contacts, sent folder, and inbox frequency. Editable by you.

Cover charge

The small payment an unknown sender pays to reach your inbox. You set the amount (default ~$0.04). The payment settles directly to your wallet.

Cashu

An open ecash protocol built on top of the Lightning Network. An ecash token is cryptographic proof that a payment was made.

ecash

A form of digital cash using blinded signatures, so payments can move between two parties without the issuer knowing who transacted. Rythm uses ecash for privacy-preserving payment verification.

Lightning Network

A payment network built on top of Bitcoin that enables fast, low-cost payments. Rythm uses Lightning for sender-to-recipient payment settlement.

Non-custodial

Rythm never holds your money or your email content. Payments flow directly from sender to your Lightning wallet. Content is scanned in-memory and discarded in milliseconds.

Mint (Cashu mint)

A public issuer of Cashu tokens. The mint holds the backing Lightning funds; users hold only the blinded-signature tokens.

Token (Cashu token)

A cryptographic proof that a payment was made to the mint. Transferable, redeemable, privacy-preserving. Rythm validates tokens and melts them instantly.

Melt (Cashu melt)

The act of redeeming a Cashu token back into a Lightning payment. Rythm melts incoming tokens directly to the user's Lightning wallet.

LNURL

A protocol for generating Lightning payment requests from a static URL. Rythm supports any LNURL-compatible wallet (Cash App, Strike, Blink, Primal, Tether Wallet).

Fail-open

A design principle: if the protection layer breaks, email delivers normally. You never miss a message because of Rythm.

Safety buffer

A small amount set aside from each payment to cover Lightning routing fees. Any unused buffer becomes 'keep the change' revenue for Rythm.

RYTHM: PAID

The Gmail/Outlook label applied to emails from unknown senders who paid the cover charge.

RYTHM: REJECTED

The label applied to emails from unknown senders who did not pay. Nothing is deleted; think of it as a waiting room.

BEC (Business Email Compromise)

A phishing attack where the attacker impersonates a known contact. Average cost per incident: $125,000. Rythm's economic gate makes mass BEC attempts economically infeasible.