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MailboxValidator Free Email Checker vs Userstack

Same instrument, two spec sheets — measured, not claimed.

APIMailboxValidator Free Email CheckerNEW—UserstackNEW—
Uptime · 30d
Uptime · 90d—%—%
Uptime · 30d—%—%
P50 · ms——
P95 · ms——
AuthapiKeyapiKey
CORSnoyes
HTTPSyesyes
Card requirednono
Commercial useunclearunclear
Data licenseUnverifiedUnverified
Free tierFree tier — API key requiredFree tier — API key required
Rate limitUnpublishedUnpublished
In directory since2026-07-052026-07-05
operationalpartialdownno data

MailboxValidator Free Email Checker vs Userstack: common questions

Which is more reliable, MailboxValidator Free Email Checker or Userstack?

On our scheduled checks, Userstack leads on measured uptime — MailboxValidator Free Email Checker at —% versus Userstack at —% over 90 days. These are our own probe results, not provider claims; the uptime bars above show the day-by-day record for both.

Do MailboxValidator Free Email Checker and Userstack need an API key?

Both ask you to authenticate — MailboxValidator Free Email Checker uses an API key and Userstack uses an API key. Each key is free to obtain; the Auth and Card-required rows above spell out the signup terms.

Can I call MailboxValidator Free Email Checker and Userstack from the browser?

Only Userstack is browser-friendly — it returns CORS headers over HTTPS. MailboxValidator Free Email Checker needs a server-side call or proxy, so factor that into which one fits a front-end project.

Are MailboxValidator Free Email Checker and Userstack free for commercial use?

MailboxValidator Free Email Checker has unclear commercial terms, and Userstack has unclear commercial terms. We track service terms and the data license as separate fields — see the Commercial use and Data license rows above, and confirm both before shipping either in a paid product.