How InboxPolicy's Free 72-Hour Re-Verification Cache Works
By Aria Pramesi, founder of InboxPolicy · Updated July 4, 2026
InboxPolicy caches every fresh verification for 72 hours. Re-verifying the same email inside that window returns from_cache instead of running SMTP checks again, and it costs 0 credits. After 72 hours the cache expires and the next check runs fresh at the normal $0.01 per-call rate.
What triggers a cache hit?
InboxPolicy caches every fresh verification result for 72 hours. The cache key is the email address. Submit that same address again inside the 72-hour window and the API returns from_cache in the response instead of re-running SMTP checks, and the call bills 0 credits.
What comes back is the original decision, unchanged: the action, the confidence score, and the SMTP evidence, just flagged as served from cache rather than freshly checked.
Why does this matter for agents re-validating lists before every send?
Agents that verify a list right before a send often hit the same address more than once in a short span, a follow-up firing hours after the first touch, a queue re-checked before each batch, a retry after a temporary SMTP failure.
- Re-verify a list before every send without paying per check each time
- Refresh a send queue several times a day at 0 marginal cost inside the 72-hour window
- Only pay again once an address has aged past the cache window
How does this compare to providers that bill every repeat lookup?
InboxPolicy is priced at $0.01 per call via x402, or from prepaid credit packs starting at $3.16 per 1,000 (Growth tier). None of that pricing includes a free re-check window beyond the 72-hour cache described here, and there is no separate free plan.
Other verification providers publish per-lookup or per-1,000 pricing with no documented free re-verification window, so a repeat check on the same address typically bills again as a new lookup.
What else is always free at InboxPolicy?
- Malformed-email rejection: invalid syntax is rejected before any SMTP check runs, 0 credits.
- Idempotent retries: resubmitting a request with the same idempotency key never bills twice.
- Cache hits within 72 hours: from_cache responses, 0 credits.
Outside of these three cases there is no free tier. The $0.01 per-call x402 price is the trial by design, free tiers attract list-cleaning abuse.
What happens once the 72-hour window expires?
Once 72 hours pass, the cached result expires. The next verification of that address runs a full fresh check, syntax, MX, and live SMTP, and bills at the standard rate: $0.01 per call via x402, or the applicable credit-pack rate ($5.00/1k Starter, $3.80/1k Builder, $3.16/1k Growth).
| Tool | Entry price | Free tier | Free re-verification window |
|---|---|---|---|
| InboxPolicy | $0.01 per call (x402), credit packs from $3.16/1k | None, pay-per-call is the trial | 72 hours, from_cache, 0 credits |
| ZeroBounce | ~$8.00/1k | 100/month | Not documented |
| Kickbox | ~$10.00/1k | One-time free credits | Not documented |
| MillionVerifier | ~$0.59-2.50/1k | Available | Not documented |
Frequently asked questions
How long does InboxPolicy cache a verification result?
72 hours from the original fresh verification. Any re-check of the same email address inside that window returns from_cache in the response and costs 0 credits. Once 72 hours pass, the cache expires and the next check runs a full fresh verification at the standard rate.
Does a cache hit cost credits or trigger a charge?
No. A cache hit returns from_cache and bills 0 credits, whether you pay per call via x402 or draw from a prepaid credit pack. You only pay again once the 72-hour window has passed and a fresh SMTP check runs.
Is the 72-hour cache the same thing as an idempotency key?
No, they are separate mechanisms. The cache keys off the email address and expires after 72 hours. Idempotency keys are identifiers you attach to a request so a retried call with the same key never bills twice, regardless of the cache window.
Do ZeroBounce, Kickbox, or MillionVerifier offer a free re-verification window like this?
No free re-verification caching window is documented for ZeroBounce (~$8.00/1k entry), Kickbox (~$10.00/1k entry), or MillionVerifier (~$0.59-2.50/1k entry). Each typically bills a repeat lookup as a new check, based on their published status-field pricing models.
What does a cached response actually contain?
The same decision computed on the original fresh verification: the action (send, send_with_caution, review, retry_later, or avoid), the confidence score, and the SMTP evidence, plus a from_cache flag showing it was served from the 72-hour cache rather than a new SMTP check.
Does the cache apply per email address or per account?
Per email address. If two workflows or agents on the same account check the identical address within 72 hours, the second call still returns from_cache at 0 credits. Checking a different address always triggers a fresh verification and the standard charge.