mailcheckr

Fast email validation with configurable checks: syntax, typo detection, domain blocklists, disposable-domain filtering, MX DNS validation, and optional SMTP RCPT probing.

This library is designed for practical validation (signup/contact workflows), not full RFC-5321/5322 edge-case compliance.

• Built with obuild
• Works in Bun and Node.js runtimes
• Designed to reduce network bottlenecks with cache, retries, and custom resolvers

What problem this solves

Most apps need more than regex. mailcheckr helps you:

• reject malformed addresses early
• prevent typo domains (hotnail.com -> hotmail.com)
• block disposable/temporary providers
• enforce your own blocked domains
• validate that a domain can actually receive mail (MX records)
• optionally probe mailbox acceptance via SMTP (no email body is sent)

Install

bun add mailcheckr

For local development in this repo:

bun install

Basic usage

import { checkEmail } from "mailcheckr";

const result = await checkEmail("someone@gmail.com");

if (result.valid) {
  console.log("valid");
} else {
  console.log(result.reasonId, result.message);
}

Disable specific checks

Turn off disposable-domain check (your request):

const result = await checkEmail("user@mailinator.com", {
  checkDisposable: false,
});

Other toggles:

await checkEmail("someone@hotnail.com", {
  checkTypo: false,
  checkBlocklist: false,
  checkVendorRules: false,
  checkMx: true, // keep DNS MX verification
});

All options

await checkEmail("user@company.com", {
  level: "dns", // "syntax" | "dns" | "deep"
  timeout: 3000,
  dnsServer: "",
  extraDisposableDomains: [],
  blocklistDomains: ["disposable-email.com"],
  checkBlocklist: true,
  checkDisposable: true,
  checkTypo: true,
  checkVendorRules: true,
  checkMx: true,
  cache: true,
  cacheTtl: 300_000,
  skipCache: false,
  usePopularMxCache: true,
  popularMxCache: {
    "company.com": ["mx.company.com"],
  },
  dohProviderUrl: "https://cloudflare-dns.com/dns-query",
  dohRetryAmount: 2,
  mxResolver: async (domain) => {
    return domain === "example.com" ? ["mail.example.com"] : [];
  },
  smtpProbe: false, // enable mailbox probe
  smtpProbeTimeoutMs: 2500, // timeout in milliseconds
  smtpProbeMaxMxHosts: 1, // keep low for speed
  smtpProbeCatchAllCheck: true,
});

usePopularMxCache helps reduce cold DNS latency for common providers by seeding known MX entries. dohProviderUrl and dnsServer are restricted to trusted allowlisted providers.

When smtpProbe is enabled, the checker performs SMTP handshake up to RCPT TO and stops before DATA (no message body sent). If SMTP probing is temporarily unavailable or timed out, the result remains valid when MX checks pass, and the smtp.status is unverifiable.

SMTP probe example (fast mode)

const result = await checkEmail("someone@gmail.com", {
  smtpProbe: true,
  smtpProbeTimeoutMs: 1500,
  smtpProbeMaxMxHosts: 1,
});

console.log(result.valid, result.smtp?.status);

Custom error messages

import {
  checkEmail,
  INVALID_REASON_AMOUNT_OF_AT,
  INVALID_REASON_USERNAME_GENERAL_RULES,
  INVALID_REASON_DOMAIN_GENERAL_RULES,
  INVALID_REASON_NO_DNS_MX_RECORDS,
  INVALID_REASON_DOMAIN_IN_BLOCKLIST,
  INVALID_REASON_USERNAME_VENDOR_RULES,
  INVALID_REASON_DOMAIN_POPULAR_TYPO,
} from "mailcheckr";

const customReasons = {
  [INVALID_REASON_AMOUNT_OF_AT]: "Email must contain exactly one @ symbol",
  [INVALID_REASON_USERNAME_GENERAL_RULES]:
    "Username contains invalid characters",
  [INVALID_REASON_DOMAIN_GENERAL_RULES]: "Domain name is invalid",
  [INVALID_REASON_NO_DNS_MX_RECORDS]:
    "Domain does not have mail server records",
  [INVALID_REASON_DOMAIN_IN_BLOCKLIST]: "This email domain is not allowed",
  [INVALID_REASON_USERNAME_VENDOR_RULES]:
    "Username does not meet provider requirements",
  [INVALID_REASON_DOMAIN_POPULAR_TYPO]:
    "Domain appears to be a typo (did you mean gmail.com?)",
};

const result = await checkEmail("someone@gmail.com");
if (!result.valid) {
  console.log(customReasons[result.reasonId!]);
}

Node.js custom MX resolver

import { resolveMx } from "dns/promises";
import { checkEmail } from "mailcheckr";

async function nodeResolver(emailDomain: string): Promise<string[] | false> {
  try {
    const records = await resolveMx(emailDomain);
    return records.map((rec) => rec.exchange);
  } catch (error) {
    const err = error as Error;
    if (err.message.includes("ENOTFOUND")) return [];
    return false;
  }
}

const result = await checkEmail("someone@gmail.com", {
  mxResolver: nodeResolver,
});

Build and test

bun run build
bun test