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.
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("hello@example.com");
// { email, action: "send" | "hold" | "kill", verdict: "OK" | "WEAK" | "DEAD", reason, checks: { syntax, mx, smtp, catch_all } }

await verifyBatch(["a@b.com", "c@d.org"], { smtp: true, helo: "probe.example.com", from: "probe@example.com" });

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.