mxprobe-core

The MX Probe engine. Zero dependencies, Node 20+. Two tiers:

1. DNS. Free and local. Finds every domain that cannot receive mail: no MX and a web host behind the A record, a null MX, an MX that does not resolve, a domain that does not exist, a parking host as MX. Forwarder MX hosts (Cloudflare Email Routing, Namecheap, ImprovMX) are a hold.
2. SMTP probe. Connects to the MX on port 25, EHLO, MAIL FROM, RCPT TO for the address and for a random local part (the catch-all test), QUIT. No message is ever sent. Needs outbound port 25, which most laptops and clouds block; the hosted API at https://api.mxprobe.dev runs it for you.

import { verify, verifyBatch, createVerifier } from "mxprobe-core";

await verify("[email protected]");
// { email, action: "send" | "hold" | "kill", verdict: "OK" | "WEAK" | "DEAD", reason, checks: { syntax, mx, smtp, catch_all } }

await verifyBatch(["[email protected]", "[email protected]"], { smtp: true, helo: "probe.example.com", from: "[email protected]" });

const v = createVerifier({ smtp: true, smtpConcurrency: 3, autoDisableSmtp: false }); // shared limits across calls

checks.smtp is skipped on the DNS tier, else accepted, rejected, refused, deferred, unreachable or dropped. Only rejected (a 5xx that names the mailbox) kills; everything else that is not accepted holds.

Options (all optional): smtp, helo, from, port, hostOverride, dnsTimeoutMs, smtpTimeoutMs, smtpConcurrency, dnsConcurrency, autoDisableSmtp, resolver (an object with resolveMx, resolve4, resolve6, reverse, for tests).

Written in TypeScript; the package ships its declarations. VerifyResult, Action, Verdict, SmtpCheck, Checks, Summary, VerifierOptions, Resolver and the hosted API's response types (SignupResponse, VerifyResponse, BalanceResponse, CheckoutResponse, ApiErrorBody) are exported as types.

MIT. Part of https://github.com/andrewchmr/mxprobe.