Knock knock.
“Who's there?” is now a paid question.
Knock is the inbox firewall AI agents can pay through. Publish a Knock address — or protect the Gmail or iCloud inbox you already have. Strangers, human or AI, post a refundable deposit to reach you. Important messages get through. Noise pays you.
Already have a Gmail or iCloud address? Protect the inbox you already have →Works with your email
Start with a fresh address you hand out, or keep the one you already use and put the gate in front of it.
Your Knock address
Claim a handle and publish it. Every stranger meets the gate; everyone you release passes free from then on.
See how it works →Guided setupYour Gmail
A forwarding address and one filter. No app access required — we never connect to your Google account, we just catch Google's verification email for you.
Protect my Gmail →Native rulesYour iCloud
One server-side rule at icloud.com forwards your mail through the gate. Set up in minutes, nothing running on your Mac.
Protect my iCloud →Set your stake
Define your minimum attention fee. Humans might pay $2; unverified AI agents might require $10. You keep the rules.
Filter at the edge
Unknown senders hit a refundable deposit challenge. Known contacts and allowlisted domains pass through untouched.
Settle with transparency
Release — from the email itself — and the deposit refunds instantly; that sender then knocks free forever. Review a message and judge it unsolicited? Claim it and keep 90%. Ignore everything? Every deposit refunds itself in 14 days.
Structurally resistant to impersonation — allowlists admit only cryptographically authenticated senders; lookalikes meet the gate. The lookalike filter.
Every deposit ends one of four ways
Release
Refunded — that sender knocks free forever.
Claim
You keep 90% — they stay unknown and pay again.
Ignore
Auto-refund in 14 days — nothing changes.
Block
Refunded if paid — then silence.
A refund you chose creates trust; a refund the clock chose creates nothing.
Recipient Dashboard
A focused command center for your high-value communications.
Active Queue
Sorted by DepositRequesting research feedback on automated attention markets for the Q3 report...
Following up on the seed round announcement and potential synergy with...
Automated cold outreach regarding your recent domain registration for...
Your attention now has an API.
An x402-compatible challenge flow: agents and humans get the same transparent rules. Card settlement is live today; USDC over x402 arrives in beta.
POST /api/public/contact/thomas
{ "subject": "...", "body": "...",
"from": { "email": "agent@example.com" } }
← 402 Payment Required
{
"protocol": "attention/0.1",
"challenge_id": "…",
"recipient": "thomas@knock.email",
"amount": 2, "currency": "USD",
"refund_policy": {
"unopened_auto_refund_days": 14,
"reply_auto_refund": false,
"review_required_to_retain": true
},
"guarantees": ["priority_placement", "settlement_status"],
"non_guarantees": ["reply", "outcome", "response_time"],
"payment_options": {
"hosted_checkout": "https://checkout.stripe.com/…",
"x402": { "status": "beta" }
},
"status_url": "https://knock.email/api/public/challenge/…"
}Try it: curl -X POST https://knock.email/api/public/contact/thomas
Protocol Standards
How are deposits refunded?
When a recipient replies or marks a message as valuable, the deposit is released back to the sender's original payment method. Unreviewed messages auto-refund after the expiry window.
Can I block specific agents or senders?
Yes. You can maintain allowlists and blocklists by email, domain, or verified agent identity. A block prevents repeat contact even if the sender attempts to pay again.
What happens to unclaimed funds?
If a message expires without a review decision, the deposit is retained by the recipient. The platform only takes a fee on retained deposits, never on refunded ones.