MX Records: How Email Routing Works

What are MX Records?

MX records are DNS records that specify which mail servers are responsible for receiving email on behalf of a domain. When someone sends an email to user@yourdomain.com, their email server looks up your domain's MX records to find where to deliver the message.

Think of MX Records as Your Email Address

Just like a physical address tells mail carriers where to deliver letters, MX records tell email servers where to deliver emails for your domain.

Without MX records, email servers wouldn't know where to send messages destined for your domain, and your emails would bounce or never arrive.

How MX Records Work

When an email is sent to your domain, the sending mail server follows these steps:

DNS Lookup - The sending server queries DNS for MX records of the recipient domain
Priority Selection - It reads the priority values and selects the server with the lowest number (highest priority)
Connection Attempt - It attempts to connect to the primary mail server
Fallback if Needed - If the primary server is unavailable, it tries the next server in priority order
Email Delivery - Once connected, the email is transferred via SMTP

Priority Values Explained

Each MX record has a priority value (also called preference). Lower numbers indicate higher priority. The sending server always tries the lowest priority number first.

example.com.    MX    10    mail1.example.com.    ; Primary (tried first)
example.com.    MX    20    mail2.example.com.    ; Secondary (backup)
example.com.    MX    30    mail3.example.com.    ; Tertiary (last resort)

If multiple servers have the same priority, the sending server typically distributes load randomly between them (round-robin).

MX Record Syntax

An MX record consists of several components:

Host/Name    Type    Priority    Mail Server    TTL
@            MX      10          mail.example.com.    3600

Component	Description
Host/Name	The domain this record applies to (@ means the root domain)
Type	Always "MX" for mail exchange records
Priority	A number (0-65535) indicating preference; lower = higher priority
Mail Server	The hostname of the mail server (must be a valid A or AAAA record)
TTL	Time-to-live in seconds; how long DNS resolvers cache this record

MX records must point to a hostname (A/AAAA record), never directly to an IP address. The hostname must end with a period (.) in zone files to indicate a fully qualified domain name.

Common MX Record Setups

Google Workspace

Google Workspace uses multiple MX records for redundancy:

@    MX    1     ASPMX.L.GOOGLE.COM.
@    MX    5     ALT1.ASPMX.L.GOOGLE.COM.
@    MX    5     ALT2.ASPMX.L.GOOGLE.COM.
@    MX    10    ALT3.ASPMX.L.GOOGLE.COM.
@    MX    10    ALT4.ASPMX.L.GOOGLE.COM.

Microsoft 365

Microsoft 365 typically uses a single MX record:

@    MX    0    yourdomain-com.mail.protection.outlook.com.

Replace "yourdomain-com" with your domain name (dots replaced by dashes).

Zoho Mail

@    MX    10    mx.zoho.com.
@    MX    20    mx2.zoho.com.
@    MX    50    mx3.zoho.com.

ProtonMail

@    MX    10    mail.protonmail.ch.
@    MX    20    mailsec.protonmail.ch.

Self-Hosted Mail Server

@    MX    10    mail.yourdomain.com.
@    MX    20    backup.yourdomain.com.

Best Practices

Use Multiple MX Records

Always have at least two MX records with different priorities for redundancy. If your primary server goes down, email will still be delivered to your backup.

Use Appropriate Priority Gaps

Leave gaps between priority values (e.g., 10, 20, 30) so you can insert new servers later without reconfiguring existing records.

Set Reasonable TTL Values

Use a TTL of 3600 seconds (1 hour) for normal operations. Lower it temporarily before making changes to speed up propagation.

Ensure A Records Exist

Every mail server hostname in your MX records must have a corresponding A (IPv4) and/or AAAA (IPv6) record.

Don't Use IP Addresses

MX records must point to hostnames, not IP addresses. This is a requirement of the RFC specification and improves flexibility.

Test After Changes

After updating MX records, test email delivery to ensure everything works correctly. DNS changes can take time to propagate.

Troubleshooting Common MX Issues

No MX Records Found

If your domain has no MX records, email will attempt to fall back to the A record (direct delivery), but most modern mail servers won't do this. Always ensure MX records are configured.

# Check MX records using dig
dig MX yourdomain.com

# Or using nslookup
nslookup -type=mx yourdomain.com

MX Points to IP Instead of Hostname

MX records cannot point directly to IP addresses. If you see this error, update the MX record to point to a hostname that has an A record.

Invalid: @ MX 10 192.168.1.1

Valid: @ MX 10 mail.example.com.

Missing A Record for Mail Server

If your MX record points to a hostname that doesn't have an A record, email delivery will fail. Verify the A record exists:

# Verify A record exists for mail server
dig A mail.yourdomain.com

DNS Propagation Delays

After changing MX records, it can take up to 48 hours for changes to propagate globally. During this time:

Some emails may go to old servers
Some emails may go to new servers
Test from multiple locations/networks

Firewall or Port Blocking

Even with correct MX records, email won't work if port 25 (SMTP) is blocked on your mail server. Verify connectivity:

# Test SMTP connectivity
telnet mail.yourdomain.com 25

# Or using nc
nc -zv mail.yourdomain.com 25

Summary

1.MX records tell other mail servers where to deliver email for your domain
2.Lower priority numbers mean higher preference (10 is tried before 20)
3.Always configure multiple MX records for redundancy and reliability
4.MX records must point to hostnames with valid A records, never IP addresses
5.Test your configuration after making changes and allow time for DNS propagation

Resources

RFC 5321 - Simple Mail Transfer Protocol Google Workspace MX Record Setup Microsoft 365 DNS Records Setup