Email validation API for signup forms

Validate the address on submit, reject only what is certainly wrong, and never lose a real user to a maybe.

A signup form has one expensive mistake: rejecting a real person. A wrong "invalid" costs you a customer and a support ticket you cannot answer. So the verifier behind a signup form has to refuse to guess.

One call on submit gives you a verdict with its reason, a safe default in suggested_action, and evidence you can read when someone writes in. Unknowns are free, so you can set a tight timeout and let the form move.

Wire it in

// On the server: the key never reaches the browser.
const res = await fetch('https://api.verimail.io/v3/verify', {
  method: 'POST',
  headers: { Authorization: `Bearer ${process.env.VERIMAIL_KEY}` },
  body: new URLSearchParams({ email, timeout: '2000', evidence: 'true' }),
});
const v = await res.json();
if (v.did_you_mean) return ask(`Did you mean ${v.did_you_mean}?`);
if (v.suggested_action === 'reject') return reject(v.reason);   // undeliverable, or disposable
createAccount(email, { confirmed: v.verdict === 'deliverable' ? 'pending' : 'required' });

What to do with each answer here

deliverable	Create the account. Send the welcome email as usual.
undeliverable	Show the reason in plain words next to the field ("that domain can't receive email"). Offer did_you_mean first when it is set.
risky	Create the account and require email confirmation before anything that costs you. A disposable address comes back suggested_action: reject; use that for free trials.
unknown	Same as risky. The server would not say, in two seconds; that is not a reason to turn a person away. Not billed.

What this does not do

It does not prove a human controls the address. Confirmation by email does; do both when the flow can wait for a click.
It does not resolve catch-all domains. Those come back risky, with the domain's reply in the evidence.
A brand-new disposable domain can pass until the daily list catches it.

Reference: the API, handling results, result codes. A free key gives you 100 verifications a month: get one.