# MX record

> An MX record is the DNS entry that names the mail servers accepting e-mail for a domain. How MX lookups work, what null MX means, and what an MX check can't tell you.

Canonical: https://mobilevalidate.com/glossary/mx-record · Last updated: 2026-09-26

![A cell tower sends signal to a SIM card; the line type is identified as mobile rather than landline or VoIP.](https://mobilevalidate.com/images/carrier-and-line-type-lookup.svg)

*Carrier lookup returns the line type (mobile, landline or VoIP) and the network behind the number.*


An MX (mail exchanger) record is a DNS record that tells the internet which servers accept e-mail for a domain, and in what order to try them. When you send to `someone@example.com`, your mail server looks up the MX records of `example.com` and delivers to the host they name. An MX check is the cheapest way to confirm that the domain part of an address can receive mail at all.

## How does an MX record work?

The record type is defined in the DNS standard [RFC 1035](https://www.rfc-editor.org/rfc/rfc1035) (§3.3.9). Each MX record has two fields: a **preference** number and an **exchange** host name. Lower preference values are tried first. A domain with two records might look like this:

```
example.com.  3600  IN  MX  10  mx1.example.com.
example.com.  3600  IN  MX  20  mx2.example.com.
```

Mail routing is specified in [RFC 5321](https://www.rfc-editor.org/rfc/rfc5321) §5.1. The sender looks up MX records for the domain, sorts them by preference, and tries each host in turn. If a domain has **no** MX record but has an address record (A or AAAA), the sender treats the domain itself as an implicit mail exchanger with preference 0.

## What is a null MX record?

Because of that fallback, a domain without MX records can still attract delivery attempts that retry for days. [RFC 7505](https://www.rfc-editor.org/rfc/rfc7505) fixes this with a **null MX**: a single MX record with "preference number 0 and a zero-length label, written in master files as '.'". It declares that the domain accepts no mail, so senders should fail immediately and tell the user.

```
no-mail.example.  3600  IN  MX  0  .
```

For validation, a null MX is a definite "this domain can't receive e-mail".

## What can an MX check tell you, and what can't it?

| Result | Meaning | Typical action |
|---|---|---|
| Domain doesn't exist (NXDOMAIN) | Almost always a typo | Ask the user to correct it |
| Null MX | Domain explicitly accepts no mail | Reject the address |
| No MX, but an A/AAAA record | Mail may still be delivered via the fallback | Accept with caution, confirm |
| Valid MX records | The domain can receive mail | Go on to further checks |

An MX lookup says nothing about the **mailbox**. `real.person@gmail.com` and `made.up.name@gmail.com` share the same MX records. A domain with good MX records may also be a [catch-all](/glossary/catch-all-email) that accepts everything, or a [disposable e-mail](/glossary/disposable-email-address) service.

## Why does it matter for deliverability and fraud?

Mistyped domains (`gmial.com`, `outlok.com`) are among the most common sign-up errors. Catching them at the form, while the person can still fix them, prevents undeliverable receipts and password resets, and avoids the [hard bounces](/glossary/email-bounce) that hurt sender reputation. For fraud, an MX check is only a first filter: fake-account tooling uses real, working domains.

## How does MobileValidate fit in?

Run syntax and MX checks yourself: they are free, fast and available in every language's DNS library. MobileValidate adds the next step for **major webmail providers**: our [mailbox check](/services/email-verification) answers whether the specific mailbox exists, without sending mail. Addresses on other domains return `unknown` with `UNSUPPORTED_PROVIDER` and are not charged. See [e-mail checks](/docs/emails) in the docs.

## Frequently asked questions

### What does an e-mail MX record check tell me?

It tells you whether the domain after the @ publishes mail servers, so mail to it can be routed. It says nothing about whether the specific mailbox before the @ exists.

### What happens if a domain has no MX record?

Senders fall back to the domain's A or AAAA address record and try to deliver there, as RFC 5321 describes. A domain that wants no mail at all can publish a null MX record, which tells senders to fail at once.

### Is an MX check enough to validate e-mail at sign-up?

No. It catches mistyped and non-existent domains cheaply, but a domain with valid MX records can still reject the mailbox, accept everything as a catch-all, or be a disposable-mail service.