InboxPolicy vs Kickbox: Which Email Verification API Fits?
By Aria Pramesi, founder of InboxPolicy · Updated July 4, 2026
Kickbox charges about $10 per 1,000 verifications, returning status fields plus a Sendex score through a dashboard workflow with one-time free credits. InboxPolicy returns a send-decision with SMTP evidence at $0.01 per call, ships an MCP server and keyless x402 payments, and re-verifies free within 72 hours.
What's the core difference between InboxPolicy and Kickbox?
Kickbox checks an email address and hands back status fields plus a Sendex score, a quality rating you read and decide on yourself. It's built dashboard-first, for a person reviewing a list.
InboxPolicy checks syntax, MX, and live SMTP against its own verification engine, then runs a deliverability policy over that evidence and returns one action: send, send_with_caution, review, retry_later, or avoid, with a confidence score and the SMTP evidence behind it. The decision is already made; an agent can act on the response without a human reading a score first.
How does pricing compare?
Kickbox's entry price is about $10.00 per 1,000 verifications.
InboxPolicy's pay-per-call price is $0.01 per fresh verification via the x402 protocol, USDC on Base, no account or API key required, which works out to roughly $10 per 1,000, similar to Kickbox's entry rate.
Where InboxPolicy pulls ahead is prepaid volume: Starter is $5 for 1,000 credits ($5.00/1k), Builder is $19 for 5,000 ($3.80/1k), and Growth is $79 for 25,000 ($3.16/1k), paid by card with the key emailed. Volume pricing is available on request. No comparable prepaid figure is published for Kickbox.
How do they handle unknown and catch-all addresses?
Roughly 30-40% of B2B email addresses sit on catch-all domains, meaning the mail server accepts anything at that domain and a single SMTP check can't tell you if the specific mailbox exists.
InboxPolicy maps unknown or catch-all results to review, it never guesses one of these as safe to send. Kickbox's specific handling of catch-all and unknown results isn't detailed in available data, its Sendex score is the signal it surfaces instead, but no confirmed methodology for catch-all resolution is available to compare directly.
Is InboxPolicy or Kickbox better for AI agents and automated workflows?
InboxPolicy is built agent-native. It ships an MCP server exposing decide_send, verify_email, batch tools, and usage, so an agent calls a tool and gets a decision back. The REST API supports idempotency keys (a repeated call with the same key never bills twice), per-item batch results, async batches up to 50,000 emails, and signed completion webhooks.
The x402 flow lets an agent skip account setup entirely: a keyless request returns an HTTP 402 with machine-readable payment terms, the agent pays $0.01 in USDC on Base via an X-PAYMENT header, and gets its decision plus an on-chain settlement receipt.
Kickbox has no MCP server and no x402 support. It's dashboard-first, built for someone uploading a CSV and reviewing results in a UI, not for an agent making a real-time send decision.
When should you choose Kickbox instead of InboxPolicy?
Choose Kickbox when the job is a dashboard-driven marketing workflow: uploading a list, reviewing Sendex scores by hand, and exporting a cleaned file, with one-time free credits to test the service before paying.
InboxPolicy isn't built for that use case, it's an API that returns a send-decision per address or per batch for an automated pipeline, not a UI for manual list review.
What free options does each offer?
InboxPolicy has no free tier and no free plan. The $0.01 per-call x402 price is deliberately the trial, free tiers attract list-cleaning abuse. What is always free, 0 credits, on every account: cache re-verification of the same address within 72 hours (returns from_cache), rejection of malformed emails before an SMTP check ever runs, and idempotent retries under the same idempotency key.
Kickbox offers one-time free credits at signup instead, a fixed allotment to test the dashboard before buying more.
| Dimension | InboxPolicy | Kickbox |
|---|---|---|
| What it returns | An action (send, send_with_caution, review, retry_later, avoid) with a confidence score and SMTP evidence | Status fields plus a Sendex quality score |
| Entry pricing | $0.01 per fresh verification via x402 (about $10 per 1,000) | About $10.00 per 1,000 |
| Prepaid packs | Starter $5 / 1,000 ($5.00/1k), Builder $19 / 5,000 ($3.80/1k), Growth $79 / 25,000 ($3.16/1k) | Not specified in available data |
| Free offering | No free tier. Cache re-verification within 72 hours, malformed-email rejection, and idempotent retries are always free (0 credits) | One-time free credits on signup |
| Agent integration | MCP server (decide_send, verify_email, batch, usage tools); keyless x402 payment flow (HTTP 402, pay $0.01 USDC on Base, get an on-chain receipt) | No MCP server, no x402 |
| Workflow style | API-first, built for automated agents making a send/no-send call | Dashboard-first, built for manual list uploads and review |
| Unknown/catch-all handling | Unknown or catch-all results map to review, never guessed as safe | Not detailed in available data |
| Best for | Agents and pipelines that need a send decision at call time | Marketing teams cleaning lists in a dashboard with a quality-score signal |
Frequently asked questions
Is InboxPolicy cheaper than Kickbox?
At the entry rate they land close together: InboxPolicy's x402 price is $0.01 per call, about $10 per 1,000, versus Kickbox's roughly $10.00 per 1,000. InboxPolicy's prepaid credit packs go lower, down to $3.16 per 1,000 on the $79 Growth pack. No prepaid or volume pricing figure is available for Kickbox, so a full pack-to-pack comparison isn't possible.
Does Kickbox have an MCP server or support x402 payments?
No. Kickbox is a dashboard-first tool with no MCP server and no x402 support. InboxPolicy ships an MCP server exposing decide_send, verify_email, batch tools, and usage, plus a keyless x402 flow where an agent gets an HTTP 402, pays $0.01 in USDC on Base, and receives its decision with a settlement receipt.
What does InboxPolicy return that Kickbox doesn't?
InboxPolicy returns one action, send, send_with_caution, review, retry_later, or avoid, with a confidence score and SMTP evidence behind it. Kickbox returns status fields plus a Sendex quality score. Both give you a signal about the address; InboxPolicy's output is a decision an agent can act on directly, Kickbox's is a score you interpret yourself.
Does InboxPolicy have a free tier like Kickbox?
No. InboxPolicy has no free tier or free plan, the $0.01 per-call x402 price is the trial. Kickbox offers one-time free credits at signup. InboxPolicy does make cache re-verification within 72 hours, malformed-email rejection, and idempotent retries free of charge, 0 credits, on every account.
Which is better for cleaning a large purchased list?
Kickbox's dashboard and CSV-upload workflow is built for that kind of one-time bulk job. If lowest cost per verification is the only goal, MillionVerifier's roughly $0.59 to $2.50 per 1,000 is the cheaper option for one-shot list cleaning. InboxPolicy is built for per-call send decisions inside an agent workflow, not bulk dashboard cleanup.
How does verification accuracy compare between the two?
InboxPolicy shows about 90% typical valid-verdict agreement with MillionVerifier across 2M+ verifications on its prior engine, though this varies by vertical and drops as low as 60% in some. No comparable published accuracy figure for Kickbox is available in current data, so its accuracy can only be described qualitatively here, not numerically.