Make one paid email verification request in five minutes

By Aria Pramesi, founder of InboxPolicy · Updated July 12, 2026

For API and agent builders: pay once, get a send decision. No account or API key—just a funded wallet and the working x402 v2 client path below.

How does the x402 payment flow work?

A keyless request to InboxPolicy's verification endpoint returns an HTTP 402 response with machine-readable payment requirements instead of a decision. Your agent reads those requirements, attaches a PAYMENT-SIGNATURE header carrying the quoted $0.006-$0.01 rate in USDC on Base, and resubmits the same request.

InboxPolicy settles the payment on-chain and returns the send decision in the same response, along with an on-chain settlement receipt. No signup step, no API key exchange, and no separate billing call.

The same decision logic is also exposed as an MCP server with decide_send, verify_email, batch tools, and a usage tool, so an agent can call it directly. The REST API adds idempotency keys, per-item batch results, async batches up to 50,000 emails, and signed completion webhooks.

What do I get back per paid verification?

Each verification runs syntax, MX, and live SMTP checks against InboxPolicy's own verification engine. A deliverability policy then turns that evidence into one action instead of a raw status field:

• send
• send_with_caution
• review
• retry_later
• avoid

The response includes a confidence score and normalized signals for catch-all, disposable, free, and role-account status, not just the action label. Across 2M+ verifications on the prior engine, typical valid-verdict agreement with MillionVerifier ran around 90%, though this varies by vertical and has run as low as 60% in some; MillionVerifier's published pricing runs from $3.90 per 1,000 at entry down to $0.45 per 1,000 at 1M volume.

Unknown or catch-all results are mapped to review, never guessed as safe. Roughly 30-40% of B2B addresses sit on catch-all domains, so this happens often, not as an edge case.

Do I need an account or API key?

No, not for the x402 path. It's keyless: no signup form, no API key issuance, no dashboard login. You send the request, get the 402, pay the wallet's quoted $0.006-$0.01 rate in USDC on Base, and get your decision.

Accounts and keys only come into play if you'd rather pay by card up front instead of per call. Prepaid credit packs are purchased by card and the key is emailed to you:

• Starter: $5 for 1,000 credits ($5.00 per 1,000)
• Builder: $19 for 5,000 credits ($3.80 per 1,000)
• Growth: $79 for 25,000 credits ($3.16 per 1,000)

Volume pricing is available on request. There is no free tier and no free plan on either path; the standard $0.01 x402 call is the trial, while repeat wallet usage earns rates down to $0.006.

Is x402 cheaper than a credit pack or a competitor?

The standard single-call x402 rate is $0.01 ($10.00 per 1,000); rolling 30-day wallet usage applies 20% off after 1,000 settled units and 40% off after 10,000. Batch calls start from the configured per-email bulk quote and receive the same earned percentage. Prepaid packs provide fixed card pricing at $3.16-$5.00 per 1,000.

Against dashboard-first competitors, entry pricing runs higher: ZeroBounce starts around $9.90 per 1,000 on subscription (or $19.50 pay-as-you-go) and Kickbox around $10.00 per 1,000, all for status-field output rather than a send decision. MillionVerifier runs from $3.90 per 1,000 at entry down to $0.45 per 1,000 at 1M volume and is the better choice if your job is a one-shot cleaning of a large scraped list rather than a live, per-send check. To project costs for your own list across x402, credit packs, and competitors, use the cost calculator.

What about verifying a list, not just one address?

Agents make one x402 payment transaction for a batch of up to 5,000 emails; the amount is still calculated per email from the configured bulk quote. POST /v1/batches/verify accepts that single batch payment instead of requiring one transaction per address.

Send the batch keyless, without a PAYMENT-SIGNATURE header, and InboxPolicy replies 402 with the total price quoted as emails × the per-email bulk rate, in USDC on Base:

curl -i -X POST https://api.inboxpolicy.com/v1/batches/verify \
  -H "content-type: application/json" \
  -d '{"emails": ["a@x.com", "b@y.com", "c@z.com"], "strictness": "default"}'
# 402 Payment Required — quotes total = emails x per-email bulk rate, in USDC

A standard x402 v2 client (@x402/core with @x402/evm) reads that quote, signs, and retries automatically. On acceptance, the same call returns 202 with a batch_id and a keyless access_token — no account needed to poll status or pull per-item results, just an x-batch-access-token header. Bigger lists get chunked client-side into multiple calls; each payment covers exactly the emails in that call.

What can be free on x402?

Two result types can avoid a charge:

• Cache re-verification: an eligible lookup of the same normalized address and strictness within 72 hours, with force_refresh false, can return from_cache and cost 0 credits.
• Malformed-email rejection: addresses rejected on syntax before SMTP is ever attempted cost 0 credits.

Keyless x402 calls do not use the API-key idempotency wrapper. After an indeterminate settlement, do not sign or submit a second payment.

When is x402 verification not the right fit?

Three cases where a different tool serves you better:

• One-shot cleaning of a huge scraped list at the lowest cost at high volume: use MillionVerifier at roughly $3.90 per 1,000 entry down to $0.45 per 1,000 at 1M.
• Spam-trap or abuse-address detection: use ZeroBounce, which maintains a dedicated spam-trap and abuse database.
• A dashboard-first workflow built around CSV uploads: use Kickbox or Emailable.

InboxPolicy is built for agents making a real-time send decision per address, with normalized signals and an on-chain receipt attached, not for bulk list hygiene at the lowest unit cost.

Frequently asked questions