MX Records Explained: What They Are and Why They Matter for Email

Every domain that can receive email has MX records configured in DNS. If those records are missing or misconfigured, email can't be delivered — period. Understanding MX records is essential for anyone debugging email delivery problems or verifying whether an address is reachable.

What is an MX record?

MX stands for Mail Exchanger. An MX record is a DNS record that maps a domain name to the mail servers responsible for receiving email on behalf of that domain.

When someone sends an email to user@example.com, the sending mail server:

1. Looks up the MX records for example.com
2. Connects to the highest-priority mail server listed
3. Delivers the message

Without MX records, the sending server has nowhere to deliver the message and returns it as a hard bounce.

What an MX record looks like

MX records have two fields: priority (a number) and value (the hostname of the mail server).

A domain might have records like:

| Priority | Mail server | |---|---| | 1 | aspmx.l.google.com | | 5 | alt1.aspmx.l.google.com | | 5 | alt2.aspmx.l.google.com | | 10 | alt3.aspmx.l.google.com | | 10 | alt4.aspmx.l.google.com |

This is a typical Google Workspace configuration. The sending mail server attempts delivery to the lowest-priority number first (1 in this case). If that server is unavailable, it tries the next ones.

The priority numbers are relative — only their ordering matters, not the specific values. 1 beats 5 beats 10.

Common MX record configurations by provider

| Email provider | Primary MX record | |---|---| | Google Workspace | aspmx.l.google.com | | Microsoft 365 | domain-com.mail.protection.outlook.com | | Zoho Mail | mx.zoho.com | | Fastmail | in1-smtp.messagingengine.com | | ProtonMail | mail.protonmail.ch | | Amazon SES | inbound-smtp.*.amazonaws.com |

If you see one of these in an MX lookup, you know which platform the domain is using for email hosting.

How to look up MX records

You can look up MX records using the free MX Lookup tool — enter any domain or email address and get the full list of MX records sorted by priority.

Alternatively, via command line:

nslookup -type=MX example.com

or on macOS/Linux:

dig MX example.com

What missing MX records mean

If a domain has no MX records, it cannot receive email. Any message sent to that domain will be rejected by the recipient's mail server with a bounce.

Common causes of missing MX records:

• The domain was registered but never configured for email
• The domain's email hosting has expired or been cancelled
• The domain has been abandoned and the company is no longer operating
• The DNS records were accidentally deleted during a migration

When you're verifying an email list, every address on a domain with no MX records will be marked invalid. There's no point attempting SMTP verification — there's no server to connect to.

Catch-all domains and MX records

A domain can have valid MX records but be configured to accept all email regardless of whether a specific mailbox exists. This is a "catch-all" configuration — the mail server returns a positive SMTP response (250 or 252) for any address, whether or not the specific inbox user@example.com actually exists.

This is why MX record lookup alone isn't sufficient to verify an individual address — you need SMTP-level verification too. The email verification process handles all three layers: syntax, DNS/MX, and SMTP.

MX records and spam filtering

MX records also affect inbound spam filtering. Services like Google Workspace route all incoming email through their filtering infrastructure at the MX record level — so by pointing your domain's MX records at Google's servers, all incoming mail is scanned before it reaches your inbox.

Some organizations use a dedicated email security gateway (like Mimecast or Proofpoint) and put those addresses in their MX records, with their actual mail provider further down the chain.

Summary

MX records tell the internet where to deliver email for a domain. Missing records mean hard bounces. Multiple records with different priorities provide redundancy. Check MX records when debugging email delivery, and verify them as part of any email list hygiene process — an address on a domain with no MX records is undeliverable by definition.