InboxPolicy vs ZeroBounce vs Kickbox vs MillionVerifier — for AI agents
Honest comparison across eight email verification providers. Different tools win different jobs. Here is where each one actually wins — and where InboxPolicy is the wrong choice.
Competitor prices and features verified July 2026 · Updated 2026-11-12.
At a glance
| InboxPolicy | ZeroBounce | Kickbox | MillionVerifier | Emailable | NeverBounce | Hunter | Clearout | Truelist | |
|---|---|---|---|---|---|---|---|---|---|
| Output | Decision + evidence | Status fields | Status + Sendex score | Status fields | Status fields | Status (5 codes) | Status + finder/confidence data | Status + AI Verdict + safe-to-send flag | Result code + confidence score |
| Entry price / 1k | $3.16–$5.00/1k packs; ~$10 ($0.01/call x402) | ~$9.90/1k sub | $10.00 | $3.90–0.45/1k | ~$6.46/1k sub; ~$7.60/1k PAYG (5k tier) | ~$8/1k PAYG (<10k, reported); $10/mo for 1k plan | $49/mo ($34/mo yr) for 2k shared credits | $5.00–$7.00/1k PAYG; $5.80–$7.67/1k low-tier sub | PAYG credits + $799–$999/mo upfront unlimited API plans |
| Pay-per-call, no account | Yes — x402, $0.01 | No | No | No | No | No | No | No | No |
| Agent-native (MCP) | Yes (7 tools, OAuth + keyless x402) | Yes (API key) | Not documented | Via Zapier MCP | No first-party MCP; agent CLI/skill | No | Yes (paid plans, subscription) | Yes (hosted & local, account credits) | Not documented |
| Re-verification cost | $0 within 72h; unresolved checks $0 | Same-file dupes & unknowns free; no 72h cache documented | Unknowns refunded; no 72h cache documented | Same-file dupes free; separate files billed; discretionary risky goodwill credits | Not specified | Not specified | Not specified | Unknown/not-found free; 72h cache not specified | Included under flat-rate plan; PAYG not specified |
| Malformed input | Free, rejected locally | Free in uploaded files (single API not specified) | Flags invalid_email (charge not specified) | Free in uploaded files (single API not specified) | Not specified | Not specified | Format check included (charge not specified) | Syntax check included (charge not specified) | Not specified |
| Honest unknowns | Preserved + explained | Scored | Scored | Resolved aggressively | Not detailed publicly | Status returned; allows at signup | Flags accept-all; advises 2–5% list cap | Catch-all marked risky under AI Verdict | Claims layered resolution; API still lists catch-all/unknown |
| Spam-trap database | No | Yes | No | No | Not specified | Not specified | Not specified | Spam-trap signals checked (DB not specified) | Spam-trap detection claimed (details not specified) |
| Best for | Agents needing a decision | Deliverability + traps | Dev-friendly ESP integration | Cheapest bulk cleaning | Dashboard bulk uploads + integrations | Bulk list cleaning + ESP/CRM sync | Email finding + verification + campaigns | List cleaning, finder & Form Guard suite | Steady high-volume monthly list validation |
Note the price row honestly: at $0.01/call via x402, InboxPolicy is not the cheapest per verification (wallet-bound rolling 30-day usage discounts drop x402 to $0.008 after 1,000 settled units and $0.006 after 10,000, and prepaid credit packs run $3.16–$5.00/1k). It wins when the call is inside an agent loop that re-checks the same addresses (free within 72h), when unresolved checks shouldn't burn credits, when you want a decision instead of a status, or when you want to skip account signup entirely with x402.
Where each one wins — and when to choose an alternative
Use InboxPolicy when…
You are building an agent that must decide whether to send. decide_send(email) returns
send / send_with_caution / review / retry_later / avoid with confidence and the SMTP
evidence — no interpretation layer to hand-roll. It ships a remote MCP server exposing seven tools (decide_send, verify_email, estimate_credits, create_verify_batch, get_batch_status, get_batch_items, and get_usage) with OAuth sign-in for Claude and ChatGPT, plus keyless x402 pay-per-call settlement on REST and MCP (decide_send and verify_email). Eligible cache hits for the same normalized address and strictness within 72 hours when force_refresh is false, malformed-email rejections before SMTP, and unresolved checks (greylisted, or unknown with no catch-all or disposable signal; x402 batches are prepaid per submitted address) cost 0 credits. Honest about catch-all and greylist: they come back as review / retry_later, never guessed as safe.
When should you choose ZeroBounce instead?
Pick ZeroBounce if you need its curated spam-trap and abuse-address database — a capability InboxPolicy does not build — or a full marketing dashboard with CSV uploads, warmup, blacklist monitoring, and enrichment.
- Entry price: around $9.90/1k on subscription (min 10,000) or $19.50/1k pay-as-you-go
- Output: status fields plus spam-trap flags
- Free tier: 100 verifications per month
- Agent access: official ZeroBounce MCP server (API key required); no x402 documented
When should you choose Kickbox or Emailable instead?
Pick Kickbox or Emailable if your team wants a dashboard-first workflow: upload a CSV, review results in a UI, export a cleaned list. InboxPolicy has no dashboard-upload flow.
- Kickbox: around $10.00 per 1,000 verifications, status fields plus a Sendex quality score, and one-time free credits at signup
- Emailable: dashboard-first marketing suite with bulk uploads and integrations, $7.60/1k pay-as-you-go or $6.46/1k subscription (at the 5,000-credit tier), plus 250 signup credits
- Agent access: neither documents a first-party MCP server or x402 (Emailable publishes an agent CLI/skill)
When should you choose MillionVerifier instead?
Pick MillionVerifier if you have one large scraped list to clean a single time and lowest cost at high volume is the deciding factor. It runs from $3.90/1k at entry ($39 for 10,000, a lower total purchase than InboxPolicy's $79 25,000-credit Growth pack at $3.16/1k) down to $0.45/1k at 1M volume, and includes a free tier.
MillionVerifier also resolves unknown or catch-all addresses more aggressively than InboxPolicy does. InboxPolicy deliberately maps unknown and catch-all results to review (or send_with_caution for catch-all under aggressive strictness) instead of guessing they are safe to send, since a large share of B2B addresses sit on catch-all domains.
The honest summary
InboxPolicy does not try to out-accuracy the incumbents on raw SMTP verification — that is a data war a solo-built engine will not win, and the underlying engine is deliberately strict. What it owns is the layer above: a decision instead of a status, agent-native access via MCP, pay-per-call with no account via x402, and free 72-hour re-verification and unresolved checks for the agents that re-check lists constantly. If that is your job, nothing else fits it. If you need trap databases, email discovery, ESP sync, flat-rate unlimited plans, or the cheapest 1M+ bulk price, use the tools in the head-to-head guides below — we will point you there ourselves.
FAQ
What is the best email verification API for AI agents?
For AI agents specifically, the differentiators are agent-native access and decision-shaped output, not raw SMTP accuracy. InboxPolicy returns a send/review/avoid decision, ships an MCP server, and lets agents pay per call with x402 (no account). ZeroBounce and Kickbox are strong general-purpose verifiers but return status fields and require dashboard signup. MillionVerifier wins on price at high volume for one-shot bulk cleaning.
InboxPolicy vs ZeroBounce — which should I use?
Use ZeroBounce if you need its curated spam-trap and abuse-address database, or a marketing dashboard. Use InboxPolicy if you are building an agent that needs a send decision (not a status), can benefit from eligible 0-credit cache hits for the same normalized address and strictness within 72 hours when force_refresh is false, or wants to pay per call with x402 rather than manage prepaid credits and API keys.
Is there an email verification API that agents can pay per call without an account?
Yes. InboxPolicy supports the x402 payment protocol: a keyless request returns HTTP 402 with payment requirements, the agent pays $0.01 in USDC on Base via a PAYMENT-SIGNATURE header, and receives its verification decision plus an on-chain settlement receipt. No signup, no API key.
What is the cheapest InboxPolicy alternative for bulk list cleaning?
MillionVerifier runs $3.90/1k at entry down to $0.45/1k at 1M volume, and it includes a free tier. InboxPolicy Growth is $79 for 25,000 ($3.16/1k), while MillionVerifier's entry is $39 for 10,000 ($3.90/1k), a lower total purchase; MillionVerifier also wins on unit price at 1M ($0.45/1k) for one-shot bulk cleaning. InboxPolicy's $0.01 per-call pricing is built for ongoing per-send checks, not one-shot bulk jobs, so it isn't the cheapest choice for that use case.
Which alternative has a free tier?
ZeroBounce offers 100 free verifications per month, MillionVerifier includes a free tier, and Kickbox gives one-time free credits (not ongoing). InboxPolicy has no free tier by design: its $0.01 per-call x402 price is the trial, since free tiers tend to attract list-cleaning abuse.
Do any of these alternatives support AI agents via MCP?
ZeroBounce publishes an official MCP server (API key required), Hunter offers an official MCP server on paid subscription plans, Clearout documents hosted and local MCP servers backed by account credits, MillionVerifier documents Verify Email via Zapier MCP, and Emailable publishes an agent CLI/skill; none of the reviewed vendor docs describe keyless x402 per-request settlement. InboxPolicy ships a remote MCP server exposing seven tools (decide_send, verify_email, estimate_credits, create_verify_batch, get_batch_status, get_batch_items, and get_usage), with OAuth sign-in for Claude and ChatGPT plus keyless x402 per-call settlement for decide_send and verify_email.
How accurate is InboxPolicy compared to these alternatives?
Directly comparable, independently audited accuracy percentages across InboxPolicy, ZeroBounce, Kickbox, MillionVerifier, and Emailable are not available; inspect InboxPolicy's 40-scenario synthetic test set at /benchmark or test live addresses at $0.01/call via x402.
What happens with catch-all or unknown addresses across these tools?
MillionVerifier resolves unknown addresses aggressively rather than leaving them unresolved. InboxPolicy maps unknown results to review; catch-all results go to review by default or send_with_caution under aggressive strictness instead of being silently marked safe, since a large share of B2B addresses sit on catch-all domains.
Head-to-head comparisons
InboxPolicy vs ZeroBounce · vs Kickbox · vs MillionVerifier · vs Emailable · vs NeverBounce · vs Hunter · vs Clearout · vs Truelist