2026-09-02

Reliability, feedback loop and billing

Cost controls, a bounce feedback loop with measured accuracy, credit packs, async settling, and Turnstile.

A shared mailbox answer cache across customers, so the same address is probed once.
Bulk runs process in chunks, and each chunk writes its own part file before results are stitched.
Provider balance alerts warn before the mailbox partner runs dry.
Six @spaw.test test addresses answer fixed verdicts at no cost and are never logged.
Every response carries X-Request-Id, repeated in meta.request_id and error.request_id.
Publishable keys accept a daily credit cap (429 KEY_SPEND_CAP_REACHED once it is spent).
A circuit breaker keeps a struggling mailbox provider from slowing every lookup.
Batch requests send their mailbox probes concurrently.
A bounce feedback API (POST /v1/email/feedback) and webhook targets for Postmark, Amazon SES, Mailgun, and SendGrid; bounces join the suppression list, deliveries clear it, and the dashboard shows measured accuracy.
smtp_checked_at, the 27th response field, records when the mailbox was last probed.
Catch-all domains are remembered for a week, and an account's own delivered addresses count as verified for 90 days.
Credit packs are sold through Stripe Checkout from the Billing page.
callback_url and callback_secret on a lookup re-check an unverified answer after 5 and 20 minutes and POST the settled verdict, signed.
Bulk uploads keep the file's own columns, add the verdict columns, and offer ?variant=deliverable|risky|undeliverable downloads.
Publishable keys can require a Cloudflare Turnstile token per lookup.