What Is an MX Record? Why It Matters for Email Deliverability

What is an MX record?

An MX record (Mail Exchange record) is a type of DNS record that specifies which mail servers are responsible for receiving email for a domain. When someone sends an email to user@example.com, the sending mail server looks up the MX records for example.com to find out where to deliver the message.

Without a valid MX record, a domain cannot receive email — and any email sent to addresses at that domain will bounce immediately.

That is why MX checks matter in both infrastructure audits and verification workflows. See how to set up SPF, DKIM, and DMARC and how to check if an email address exists for the two most common operational use cases.

How MX records work

An MX record has two components:

• Priority (preference value) — a number that determines which mail server to try first. Lower numbers have higher priority. If the primary server is unavailable, the sending server tries the next lowest priority.
• Mail server hostname — the fully qualified domain name (FQDN) of the mail server that will accept the email (e.g. mail.example.com or aspmx.l.google.com for Google Workspace).

A typical MX record setup for Google Workspace looks like this:

example.com.  3600  IN  MX  1   aspmx.l.google.com.
example.com.  3600  IN  MX  5   alt1.aspmx.l.google.com.
example.com.  3600  IN  MX  5   alt2.aspmx.l.google.com.
example.com.  3600  IN  MX  10  alt3.aspmx.l.google.com.
example.com.  3600  IN  MX  10  alt4.aspmx.l.google.com.

Five records with different priorities provide redundancy — if the primary server (priority 1) is down, email is automatically delivered to the next available server.

Why MX records matter for email senders

Before sending: MX verification as a pre-send check

Before sending email to any address, your mail server (or email verifier) checks whether the domain has valid MX records. If there are no MX records, the domain cannot receive email and any send will result in a hard bounce. This is the second check performed by ListEmailCheck — after syntax validation and before the SMTP handshake.

Catching invalid MX records during verification prevents hard bounces before they affect your sender reputation.

Deliverability: your own MX records

When recipients' mail servers perform reverse lookups on your sending domain, having properly configured MX records is part of the trust signal. A sending domain with no MX records looks unusual and may be flagged as suspicious by some filtering systems.

How to look up MX records

You can look up any domain's MX records using free command-line tools or web-based checkers:

Command line (macOS/Linux)

dig MX gmail.com
# or
nslookup -type=MX gmail.com

Windows

nslookup
set type=MX
gmail.com

Free web tools

• MXToolbox MX Lookup — shows all MX records for a domain with priority values and server response times
• Google MX Checker — validates MX records specifically for Google Workspace setups

Common MX record problems and how to fix them

• No MX records at all — the domain cannot receive email. Check your DNS provider and add the records for your mail host. Until this is fixed, all email to that domain will bounce.
• MX record pointing to a non-existent hostname — the MX record exists but the hostname it points to has no A record or is unreachable. This causes timeouts and deferred delivery.
• MX record pointing to an IP address instead of a hostname — RFC 5321 requires MX records to point to hostnames, not IP addresses. This is a misconfiguration that some servers reject.
• Incorrect priority order — if your backup server has a lower priority number than your primary, email may route to your backup first, causing delays or missed messages.

MX records vs. A records vs. SPF records

• MX record — tells senders where to deliver email for your domain (inbound routing)
• A record — maps a hostname to an IP address; MX records point to hostnames, which then need A records
• SPF record — a TXT record that specifies which servers are authorised to send email from your domain (outbound authentication)

Key takeaways

• An MX record tells mail servers where to deliver email for a domain — without it, the domain can't receive email
• During email list verification, invalid MX records are identified before you send, preventing hard bounces
• Always verify your own MX records when setting up a new sending domain or migrating mail providers
• Use MXToolbox to check MX records, diagnose problems, and verify correct configuration
• ListEmailCheck checks MX records as part of every verification — ensuring addresses on invalid domains are flagged before you send.