The honest answer, before the checklist
You can wire up syntax, MX, mailbox, and disposable-domain checks yourself, in whatever order your signup flow needs, and the rest of this page shows you how. Or you can tell your agent what you're trying to keep off your list and let it stage the check. I think that second path is where this is heading: Claude, Cursor, or Codex reading your audience and staging the validation and the send together, with you approving the charge before anything runs. That's the platform Nitrosend was built to be, and it's why we're a listed app in the ChatGPT app marketplace rather than just an API with a validation endpoint.
What an email validation API checks
An email validation API verifies an address programmatically, before you send to it. The checks run in layers: syntax, then whether the domain has a mail server behind it, then whether the mailbox actually exists and can receive.
Syntax. Is it shaped like an address at all. This is the only check most signup forms do, and it is nowhere near enough.
Domain and MX records. Does the domain exist, and does it have a mail server. Catches the classic typo: gmal.com instead of gmail.com.
Mailbox existence. Does the inbox actually exist and accept mail, checked without sending anything.
Disposable and role detection. Flags throwaway domains and role addresses like info@ that never convert and often complain.
If you explicitly call validation from your signup flow, it can catch typos while the user is still there and keep bad addresses out of your list from day one. Nitrosend does not start a paid validation operation automatically on signup, import, schedule, or send.
Why bounces wreck you
At my first email company we didn't clean our list properly. We assumed people would eventually engage. They didn't, and our sending performance paid for it. That mistake is why I have opinions about this now.
The industry average bounce rate is around 0.02 percent. Not 2 percent. When your bounces climb past that, mailbox providers read it as a signal you don't know your own list, and they start filing you under spam. I've watched a properly cleaned list drop a broken send from a 22.8 percent bounce rate to 0.20 percent. Same product, same copy. The only difference was validation before the send.
And validation is not a one-off. Every three months, remove the contacts who haven't opened or purchased in the last year. Unengaged addresses decay into dead ones, and dead ones bounce.
We police this on our own platform. I once flagged a two-day-old free account about to push 23,632 cold emails through a fresh sending reputation. That send would have burned the domain before the account was a week old. Validation and warmup are not compliance theater. They are the difference between a sending reputation and a smoking crater.
The platform enforces it for you too: cross a 2 percent bounce rate and sending auto-pauses while we look at what happened. The threshold is smarter than a bare percentage, because two dead addresses in a 57-send campaign is noise, not a crisis. The pause only fires when the rate breach comes with a minimum absolute count of bounces behind it.
Where validation belongs
Most of the industry sells validation as a separate list-cleaning product. You export your list, upload it to a verifier, pay per thousand, download the results, and import them back. Every quarter, forever.
Nitrosend treats validation as an explicit prepaid operation over an exact audience. Quote first, inspect the candidate and maximum-charge counts, then start the operation with an idempotency key. Results remain attached to the selected contact channels; contact-profile enrichment is a separate operation. No plan includes a validation allowance, and validation never creates consent or increases sender capacity. Validation is still one of the cheapest levers on bounce rate, and it helps keep you off blacklist monitoring.
curl -X POST https://api.nitrosend.com/v1/my/validation_operations/quote \ -H "Authorization: Bearer $NITROSEND_API_KEY" \ -H "Content-Type: application/json" \ -d '{"list_id": 42}' curl -X POST https://api.nitrosend.com/v1/my/validation_operations \ -H "Authorization: Bearer $NITROSEND_API_KEY" \ -H "Content-Type: application/json" \ -H "Idempotency-Key: validation-list-42-2026-08" \ -d '{"list_id": 42}' → { "operation_id": "...", "status": "held" }
Go deeper
FAQ
What does an email validation API check?
It verifies an address programmatically before you send: syntax, whether the domain has a mail server behind it (MX records), whether the mailbox actually exists and accepts mail, and whether the address is disposable or role-based.
Why do bounces hurt sender reputation?
The industry average bounce rate is around 0.02 percent. When your bounces climb past that, mailbox providers read it as a signal you don't know your own list and start filing you under spam.
How often should I clean my email list?
Every three months, remove the contacts who haven't opened or purchased in the last year. Unengaged addresses decay into dead ones, and dead ones bounce.