Email verification API for Node.js

Get an API key

Keys are issued on request. Ask for one and we will set you up with free test credits. Every request sends the key as Authorization: Bearer your_key. Keep it in an environment variable, as the examples do, and out of source control. Credit prices are on the pricing page and credits never expire.

What you need

Node.js 18 or newer (for built-in fetch). No npm packages. Save the files with the .mjs extension so top-level await works.

Node 18 ships fetch and AbortSignal.timeout, so the examples need no axios or node-fetch. fetch does not throw on a 4xx or 5xx, which is why the helper checks res.ok itself. If you are on CommonJS, wrap the top-level code in an async function and call it. If you need many checks in parallel, keep concurrency modest and let the 429 retry absorb bursts.

Verify one address in Node.js

Save this as mx-one.mjs. It defines a small call helper that sends the key, retries on 429, and turns error responses into an exception or error value with the message from the API. Then verify_one posts the address and handles the one tricky case: a slow mail server.

import { readFileSync } from "node:fs";

const BASE = "https://mailexchange.com/api/v1";
const API_KEY = process.env.MAILEXCHANGE_API_KEY;
const sleep = (ms) => new Promise((resolve) => setTimeout(resolve, ms));

class MXError extends Error {
  constructor(status, message) {
    super(`HTTP ${status}: ${message}`);
    this.status = status;
  }
}

// One API call. Retries 429 with exponential backoff, throws MXError on other errors.
async function call(method, path, body, retries = 4) {
  for (let attempt = 0; ; attempt++) {
    const res = await fetch(BASE + path, {
      method,
      headers: { Authorization: `Bearer ${API_KEY}`, "Content-Type": "application/json" },
      body: body === undefined ? undefined : JSON.stringify(body),
      signal: AbortSignal.timeout(45_000),
    });
    if (res.status === 429 && attempt < retries) {
      await sleep(2 ** attempt * 1000);
      continue;
    }
    const data = await res.json().catch(() => ({}));
    if (!res.ok) throw new MXError(res.status, data.error ?? res.statusText);
    return { status: res.status, data };
  }
}

// Turn a result into an action: send, separate, skip, remove or retry.
function decide(result) {
  switch (result.state) {
    case "deliverable":
      return "send";
    case "risky":
      // low_deliverability = catch-all domain. low_quality = role address, or disposable.
      return result.reason === "low_deliverability" ? "separate" : "skip";
    case "undeliverable":
      return "remove";
    default:
      return "retry"; // unknown, or still pending
  }
}

async function verifyOne(email) {
  const { status, data } = await call("POST", "/verify", { email });
  if (status !== 202) return data;
  // 202: the mail server is slow. The check is running and already paid for.
  // Do not post the address again, poll the list instead.
  for (let i = 0; i < 36; i++) {
    await sleep(5000);
    const { data: page } = await call("GET", `/lists/${data.list_id}/results?limit=1`);
    const row = page.results[0];
    if (row && row.state !== "pending") return row;
  }
  return data; // still pending after 3 minutes: look again later with the same list_id
}

try {
  const res = await verifyOne(process.argv[2]);
  console.log(res.email, res.state, res.reason, "->", decide(res));
} catch (err) {
  console.error("Mail Exchange error:", err.message);
  process.exit(1);
}

Run it:

MAILEXCHANGE_API_KEY=mx_live_xxx node mx-one.mjs [email protected]

Typical output looks like [email protected] deliverable accepted_email -> send.

Read the result and decide what to do

A result is a flat JSON object: email, state, reason, score, and the flags accept_all, role, free, disposable, plus mx. The state is what you branch on. The reason tells you why, and matters for the risky group. The decide function in the code above maps it like this:

The function is deliberately small so you can change it. A cold email team might send separate addresses only from established inboxes, as described in catch-all emails and cold email. A signup form might accept risky addresses but block undeliverable ones. The full list of states is in email verification statuses, and how it works explains the six checks behind them.

Verify a list in Node.js

For more than a handful of addresses, submit one list instead of looping over /verify. You get a list ID straight away, poll GET /lists/{id} until its status is done (or failed), then page through /lists/{id}/results 1,000 rows at a time. Duplicates inside the list are removed and not charged. Save this as mx-list.mjs with one address per line in emails.txt.

import { readFileSync } from "node:fs";

const BASE = "https://mailexchange.com/api/v1";
const API_KEY = process.env.MAILEXCHANGE_API_KEY;
const sleep = (ms) => new Promise((resolve) => setTimeout(resolve, ms));

class MXError extends Error {
  constructor(status, message) {
    super(`HTTP ${status}: ${message}`);
    this.status = status;
  }
}

// One API call. Retries 429 with exponential backoff, throws MXError on other errors.
async function call(method, path, body, retries = 4) {
  for (let attempt = 0; ; attempt++) {
    const res = await fetch(BASE + path, {
      method,
      headers: { Authorization: `Bearer ${API_KEY}`, "Content-Type": "application/json" },
      body: body === undefined ? undefined : JSON.stringify(body),
      signal: AbortSignal.timeout(45_000),
    });
    if (res.status === 429 && attempt < retries) {
      await sleep(2 ** attempt * 1000);
      continue;
    }
    const data = await res.json().catch(() => ({}));
    if (!res.ok) throw new MXError(res.status, data.error ?? res.statusText);
    return { status: res.status, data };
  }
}

// Turn a result into an action: send, separate, skip, remove or retry.
function decide(result) {
  switch (result.state) {
    case "deliverable":
      return "send";
    case "risky":
      // low_deliverability = catch-all domain. low_quality = role address, or disposable.
      return result.reason === "low_deliverability" ? "separate" : "skip";
    case "undeliverable":
      return "remove";
    default:
      return "retry"; // unknown, or still pending
  }
}

async function verifyList(emails) {
  const { data: created } = await call("POST", "/lists", { emails, name: "node-example" });
  let summary;
  for (;;) {
    ({ data: summary } = await call("GET", `/lists/${created.id}`));
    console.log(`${summary.checked}/${summary.total} checked (${summary.status})`);
    if (summary.status === "done" || summary.status === "failed") break;
    await sleep(5000);
  }
  if (summary.status === "failed") throw new Error(`list ${created.id} failed`);

  const results = [];
  for (let offset = 0; ; offset += 1000) {
    const { data } = await call("GET", `/lists/${created.id}/results?offset=${offset}&limit=1000`);
    results.push(...data.results);
    if (data.results.length < 1000) break;
  }
  return { summary, results };
}

try {
  const emails = readFileSync(process.argv[2], "utf8")
    .split(/\r?\n/).map((s) => s.trim()).filter(Boolean);
  const { summary, results } = await verifyList(emails);

  console.log("counts:", summary.counts, "duplicates removed:", summary.duplicates_removed);
  const buckets = {};
  for (const row of results) (buckets[decide(row)] ??= []).push(row.email);
  for (const [action, addresses] of Object.entries(buckets)) console.log(action, addresses.length);

  await call("DELETE", `/lists/${summary.id}`); // optional: remove the list and its results
} catch (err) {
  console.error("Mail Exchange error:", err.message);
  process.exit(1);
}

MAILEXCHANGE_API_KEY=mx_live_xxx node mx-list.mjs emails.txt

Instead of polling you can pass a callback_url and let us call you when the list is done, see webhooks and callbacks. The whole flow, including CSV download and deleting a list, is in verify a list with the API.

Errors, 402 and 429

Every error response is JSON with an error field, which the helper surfaces in the exception message. The ones to plan for:

• 429 means you hit the per-key rate limit. The helper waits 1, 2, 4 and 8 seconds and tries again, which is safe because a rate-limited request does no work and costs nothing.
• 402 means you do not have enough credits for the request. Retrying will not help. Stop, alert someone, and top up. GET /account returns your balance if you want to check first.
• 401 means the key is missing or wrong. 400 means the request was malformed, for example no email, or a list over 100,000 addresses.

Details and a status code table are in errors, credits and rate limits.

Questions

Is there an official Node.js SDK for the Mail Exchange API?

No, and none is needed. The API is plain HTTPS and JSON, so the examples on this page use only the standard library.

How do I get an API key?

Keys are issued on request through the contact page, and they come with free test credits. Send the key as a bearer token in the Authorization header.

What should I do when /verify returns 202?

Do not send the address again. The check is already running and paid for. Read the result from /lists/{list_id}/results using the list_id in the 202 response, until the state is no longer pending.

Related

• Email verification API reference
• Verify a list with the API
• Webhooks and callbacks
• Errors, credits and rate limits
• Pricing and all developer guides
• Email verification API for Python
• Email verification API for PHP
• Email verification API for Ruby
• Email verification API for Go
• Email verification API for Java
• Email verification API for C#