Verify email syntax, MX DNS deliverability
Verify email syntax, MX DNS deliverability, disposable email detection, and role account identification
Request
- x-api-key: <your-api-key>
- Content-Type: application/json
Response
{
"status": "success",
"email": "ada@example.com",
"valid": true,
"checks": {
"syntax": true,
"domainExists": true,
"mxRecords": [
"mx1.example.com",
"mx2.example.com"
],
"disposable": false,
"roleAccount": false,
"freeProvider": false
},
"suggestion": null,
"score": 0.96,
"tokensDeducted": 1
}What it does
Four checks in one call: RFC-compliant syntax, that the domain resolves and publishes MX records, that it is not a known disposable-address provider, and whether the local part is a role account (info@, sales@). A near-miss domain gets a suggestion. No message is sent and no mailbox is probed, so nothing leaks to the address owner.
When to use it
At sign-up, before a newsletter send, when importing a list — anywhere a bounce would cost more than a token.
Request
Body
| Field | Type | Required | Description |
|---|---|---|---|
email | string | yes | The email address string to verify |
Example request
{
"email": "ada@example.com"
}Response
| Field | Type | Description |
|---|---|---|
dnsDetails | object | |
↳ hasAFallback | boolean | |
↳ hasMxRecords | boolean | |
↳ mxRecords | array<object> | |
↳ primaryMxServer | string | |
emailDetails | object | |
↳ domain | string | |
↳ localPart | string | |
↳ normalized | string | |
↳ rawInput | string | |
isValid | boolean | |
status | string | |
tokensDeducted | integer | |
validationResult | object | |
↳ isDeliverable | boolean | |
↳ isDisposable | boolean | |
↳ isFreeProvider | boolean | |
↳ isRoleAccount | boolean | |
↳ isSyntaxValid | boolean | |
↳ reason | string |
Example response
{
"status": "success",
"email": "ada@example.com",
"valid": true,
"checks": {
"syntax": true,
"domainExists": true,
"mxRecords": [
"mx1.example.com",
"mx2.example.com"
],
"disposable": false,
"roleAccount": false,
"freeProvider": false
},
"suggestion": null,
"score": 0.96,
"tokensDeducted": 1
}Errors
Every error answers with the same envelope: an HTTP status, a machine code and a sentence.
400 | The body or a parameter failed validation. The details array names the field. |
401 | No key, a revoked key or a wrong header. Send x-api-key. |
402 | The wallet cannot cover this call. Top up or wait for the monthly grant. |
404 | The resource does not exist, or it is not yours. Both answer identically and cost nothing. |
429 | Over the per-minute quota of your tier. Back off and retry. |
5xx | Upstream refused or timed out. The call is not billed. |
Code
curl -X POST 'https://app.castomapi.com/v1/verify/email' \
-H 'x-api-key: <your-api-key>' \
-H 'Content-Type: application/json' \
-d '{"email": "ada@example.com"}'const res = await fetch('https://app.castomapi.com/v1/verify/email', {
method: 'POST',
headers: { 'x-api-key': '<your-api-key>', 'Content-Type': 'application/json' },
body: JSON.stringify({"email": "ada@example.com"})
});
const data = await res.json();
console.log(data);import { request } from 'node:https';
// Node 18+ has fetch built in
const res = await fetch('https://app.castomapi.com/v1/verify/email', {
method: 'POST',
headers: { 'x-api-key': '<your-api-key>', 'Content-Type': 'application/json' },
body: JSON.stringify({"email": "ada@example.com"})
});
const data = await res.json();
console.log(data);import requests
res = requests.post(
'https://app.castomapi.com/v1/verify/email',
headers={'x-api-key': '<your-api-key>'},
json={"email": "ada@example.com"},
timeout=30,
)
print(res.json())<?php
$ch = curl_init('https://app.castomapi.com/v1/verify/email');
curl_setopt_array($ch, [
CURLOPT_CUSTOMREQUEST => 'POST',
CURLOPT_RETURNTRANSFER => true,
CURLOPT_HTTPHEADER => ['x-api-key: <your-api-key>', 'Content-Type: application/json'],
CURLOPT_POSTFIELDS => json_encode(['email' => 'ada@example.com']),
]);
$data = json_decode(curl_exec($ch), true);
print_r($data);Cost and limits
This operation costs 1 token. A call that fails is never billed. Every account gets 1000 free tokens a month; paid balances never expire.
Rate limits per minute
| No key | 10 |
| Free | 60 |
| Pro | 300 |
| Enterprise | 1200 |
Questions
What does Verify email syntax, MX DNS deliverability cost?
1 token per successful call, debited from your wallet after the response. Failed calls are free. Every account receives 1000 free tokens a month.
How do I authenticate?
Send your key in the x-api-key header. One key covers every metered endpoint; there is nothing to enable per endpoint.
Can I send an Authorization: Bearer token instead?
Not for a metered endpoint — those take the x-api-key header and nothing else. Bearer tokens belong to the account side: the dashboard and account APIs accept a JWT session, and castomapi-auth issues those through a standard OAuth2 handshake, with authorization code, client credentials and refresh grants. The auth service page covers it.
What are the limits?
10 requests a minute per IP with no key at all, 60 a minute on the free tier, 300 on Pro, 1200 on Enterprise. Above the no-key tier the quota is per account, across every endpoint.
OpenAPI: https://app.castomapi.com/docs/json