Pay for Email Verification with x402, No Account Required
By Aria Pramesi, founder of InboxPolicy · Updated July 4, 2026
No account, no API key: request verification and InboxPolicy replies with HTTP 402 and machine-readable payment requirements. Your agent pays $0.01 in USDC on Base through an X-PAYMENT header, then gets a send decision, confidence score, SMTP evidence, and an on-chain settlement receipt back in the same response.
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 an X-PAYMENT header carrying $0.01 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 for $0.01?
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:
sendsend_with_cautionreviewretry_lateravoid
The response includes a confidence score and the underlying SMTP evidence, 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.
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 $0.01 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 $0.01 x402 call is the trial, by design, since free tiers tend to attract list-cleaning abuse rather than real evaluation.
Is x402 cheaper than a credit pack or a competitor?
Per call, x402 is $0.01, or $10.00 per 1,000 verifications if you never batch into a credit pack. That's a fair price for occasional or agent-triggered calls where you don't want to hold a balance. For steady volume, the prepaid packs bring the per-verification cost down to $3.16-$5.00 per 1,000.
Against dashboard-first competitors, entry pricing runs higher: ZeroBounce starts around $8.00 per 1,000 and Kickbox around $10.00 per 1,000, both for status-field output rather than a send decision. MillionVerifier is priced lower, around $0.59-$2.50 per 1,000, 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.
What's always free, even on x402?
Three things never cost a credit or a payment, regardless of which path you use:
- Cache re-verification: re-checking the same address within 72 hours returns
from_cacheand costs 0 credits. - Malformed-email rejection: addresses rejected on syntax before SMTP is ever attempted cost 0 credits.
- Idempotent retries: resubmitting a request with the same idempotency key never bills twice.
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 possible cost: use MillionVerifier.
- 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 SMTP evidence and an on-chain receipt attached, not for bulk list hygiene at the lowest unit cost.
| Option | Price | Account or key needed | Output | Best for |
|---|---|---|---|---|
| InboxPolicy x402 | $0.01 per call (USDC on Base) | No account, no API key | Action, confidence score, SMTP evidence, on-chain receipt | Agents paying per call |
| InboxPolicy credit packs | $3.16-$5.00 per 1,000 | Card payment, key emailed | Action, confidence score, SMTP evidence | Steady volume, paid by card |
| ZeroBounce | ~$8.00 per 1,000 entry | Account, dashboard | Status fields | Spam-trap and abuse-address detection |
| Kickbox | ~$10.00 per 1,000 | Account, dashboard | Status fields plus Sendex quality score | Dashboard-first review |
| MillionVerifier | ~$0.59-$2.50 per 1,000 | Account (free tier available) | Status fields | Lowest-cost one-shot bulk cleaning |
| Emailable | Not disclosed | Account, dashboard | Status fields (bulk suite) | CSV upload workflows |
Frequently asked questions
What is x402 and how does InboxPolicy use it?
x402 is a protocol for paying per API call in USDC on Base. A keyless request to InboxPolicy returns HTTP 402 with machine-readable payment requirements instead of an error. The agent attaches an X-PAYMENT header with $0.01 in USDC, resubmits, and receives the send decision plus an on-chain settlement receipt in the same call.
Do I need to create an account to use x402 verification?
No. The x402 path is keyless: no signup, no API key, no dashboard login. You send a request, get a 402 with payment terms, pay $0.01 in USDC on Base via the X-PAYMENT header, and get your decision back. Accounts and emailed keys only apply to the prepaid credit packs (Starter, Builder, Growth), which are paid by card instead.
How much does each verification cost?
Each fresh x402 verification costs $0.01, paid in USDC on Base per call. Prepaid credit packs paid by card work out cheaper per verification at volume: Starter is $5.00 per 1,000, Builder is $3.80 per 1,000, and Growth is $3.16 per 1,000. Cache hits, malformed-email rejections, and repeated idempotency keys cost 0 credits.
What does InboxPolicy return instead of a status field?
InboxPolicy runs syntax, MX, and live SMTP checks against its own verification engine, then a deliverability policy turns that evidence into one action: send, send_with_caution, review, retry_later, or avoid, with a confidence score and the underlying SMTP evidence attached, rather than a raw valid/invalid/unknown status field.
Why is there no free tier?
InboxPolicy has no free plan; the $0.01 x402 call is the trial. This is deliberate: free tiers are commonly abused for one-shot list cleaning without ever converting to a paying workflow. Cache re-verification within 72 hours, malformed-email rejection, and idempotent retries remain free regardless of plan.
When should I use a different tool instead of x402 verification?
Use MillionVerifier for one-shot cleaning of a huge scraped list at the lowest cost. Use ZeroBounce if you need spam-trap and abuse-address detection, which it maintains a dedicated database for. Use Kickbox or Emailable if you want a dashboard-first suite built around CSV uploads rather than an API.