Rythm filters email with two user-controlled layers. Layer 1 is identity: Rythm builds a guest list of people you already know from your contacts, sent folder, starred messages, and inbox frequency. Layer 2 is cost: anyone not on the guest list sees a polite rejection notice with a link to pay a small cover charge (default about four cents). Known senders walk in; unknown senders either pay or get held for review.

How does the guest list get built?

Rythm runs a four-phase scan on connect: starred/flagged messages (people you explicitly value), your contacts book, the sent folder (anyone you have ever emailed), and inbox frequency (anyone who emails you regularly). Total time: about 8 to 10 minutes. After that, the guest list maintains itself; rescue from REJECTED once and the sender is added forever.

What happens to unknown senders?

They receive a polite rejection notice explaining that the recipient uses Rythm and including a one-tap link to pay the cover charge. If they pay, the email is delivered labeled RYTHM: PAID and the payment settles to the recipient's wallet. If they ignore it, the email stays in the RYTHM: REJECTED folder, which is a waiting room, not a trash can.

What is Cashu and why does Rythm use it?

Cashu is an open ecash protocol built on top of the Lightning Network. It lets a sender pay once and attach a cryptographic payment proof that the recipient can redeem directly, without Rythm ever holding the money. This is what makes Rythm non-custodial by design.

Rythm is a dual-layer deterministic email filter for Gmail and Outlook. Known senders walk in with no extra cost; unknown senders pay a small cover charge (default about four cents) that settles directly to your wallet. Non-custodial by design, powered by the Cashu ecash protocol over the Lightning Network.

Is Rythm non-custodial?

Yes. Rythm never stores your email content and never holds your funds. Cashu tokens are validated in-memory for milliseconds, then discarded. Payments flow from sender, to a public Cashu mint, to your Lightning wallet.

Does Rythm try to detect AI-generated email?

No. Rythm is a bouncer, not a lie detector. The cover charge creates intention: a sender who paid $0.04 has signaled that reaching you was worth it, whether a human or AI wrote the text. Rythm filters on intention, not content.

How Rythm works: the bouncer, the guest list, and the cover charge.

Rythm is an email paywall for Gmail and Outlook. A bouncer checks two things at the door: are you on the guest list, and if not, will you pay the cover? Both are set by you. No guessing. Setup takes about 12 minutes.

The guest list

Your guest list is the list of people who reach your inbox at no extra cost. Rythm builds it for you in four phases as soon as you connect your account:

Starred & flagged. Anyone you’ve ever starred (Gmail) or flagged (Outlook) is automatically in. These are senders you’ve already told your email client matter.
Contacts. Everyone in your address book. If you saved them, they’re approved.
Sent folder. Anyone you’ve ever emailed. If you’ve reached out, you want to hear back.
Inbox frequency. Senders who have emailed you back and forth with some regularity. Relationship inferred from your own behavior, not a prediction.

You can see the entire list. You can edit it. You can rescue anyone. Drag an email from the RYTHM: REJECTED folder back to your inbox and that sender is added to your guest list for good.

There’s also a global essential-services bypass: banks, courts, the IRS, 2FA providers, shipping notifications. They always get through, verified authentic.

The cover charge

Anyone not on your guest list sees a rejection notice with a link: here’s how to reach this person’s inbox, a small cover charge. You set the amount. The range is 50 to 10,000 sats (roughly $0.03 to $6); the default is ~$0.04.

The math is the point. At four cents, a real person with a real reason to reach you won’t blink. At spam scale, it’s catastrophic: a campaign of 100,000 emails would cost $4,000 out of pocket. Any cost at all destroys mass outreach economics, because mass outreach only works when it’s free.

Paid emails are delivered to your inbox with the label RYTHM: PAID. Unpaid emails from unknowns are moved to the RYTHM: REJECTED folder. Nothing is deleted. Think of it as a waiting room. Check it whenever you want; rescue whomever you want.

Setup in three steps

Step 1. Connect your inbox

Sign in with Google or Microsoft OAuth. Rythm requests only the minimum permissions needed to filter unknown senders and send rejection notices. Your email content is never stored on our side. Takes about 10 seconds.

Step 2. Your guest list builds itself

Rythm scans starred/flagged emails, contacts, sent folder, and inbox frequency to auto-build your guest list. The four-phase scan runs in the background and typically finishes in 8–10 minutes. When it’s done, everyone you already know keeps reaching your inbox with no friction.

Step 3. Set your cover charge

Pick the amount unknown senders pay to reach you. Default is ~$0.04, trivial for a real person, catastrophic at spam scale. Connect a wallet (Cash App, Strike, Blink, Primal, or Tether Wallet). The money lands in your wallet in seconds.

Under the hood (the plumbing)

When an unknown sender pays, a small payment proof rides along with the email. Rythm checks for it, confirms the payment, and the money lands in your wallet in seconds. We never hold it and we never store your email content. The scan happens in memory and everything else is discarded.

For the technically curious: Rythm uses the Cashu ecash protocol over the Lightning Network to move the sender’s payment directly to your wallet. If anything breaks on our end, your email still delivers normally.

Rythm is email processing software. It is not a cryptocurrency service, payment processor, money transmitter, or custodian of funds or tokens.

FAQ

Does Rythm work with Gmail?

Yes, any Gmail or Google Workspace account. Connect via Google OAuth.

Does Rythm work with Outlook?

Yes, any Outlook or Microsoft 365 account. Connect via Microsoft OAuth.

How long does onboarding actually take?

Around 12 minutes end-to-end: 10 seconds for OAuth, 8–10 minutes for the guest list scan, a minute or two to pick your cover charge and wallet.

What wallets are supported?

Any wallet that supports a standard payment address. We recommend Cash App, Strike, Blink, Primal, or Tether Wallet. Most take 2 to 10 minutes to set up.

What happens if I miss something important in RYTHM: REJECTED?

Nothing is deleted. The folder is a waiting room. Drag an email back to your inbox and that sender is added to your guest list for good.

What happens when I cancel?

Rythm stops processing your email, the labels can be removed automatically, and your inbox returns to exactly how it was. No lock-in, no lingering changes.

Known senders walk right in. Everyone else pays the cover.

$1.65 a month. 12 minutes to set up. Cancel anytime.

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