Understanding MX records: what they are, how they route email, MX record priority, and how to configure MX records correctly for email delivery.

MX Records Explained: How Email Routing Works

What is an MX Record?

An MX (Mail Exchange) record is a DNS record that specifies which mail server(s) are responsible for receiving email for a domain. It's the fundamental mechanism that enables email delivery across the internet.

How MX Records Work

When someone sends you an email:

Sender mail server looks up MX records for your domain
DNS query returns one or more MX records
Priority check: Server tries highest priority (lowest number) first
Connection: Attempts SMTP connection to mail server
Delivery: Email delivered if connection succeeds
Fallback: Tries next priority if first server fails

MX Record Structure

example.com. MX 10 mail1.example.com.
example.com. MX 20 mail2.example.com.

Components:

example.com.: Domain receiving email
MX: Record type
10, 20: Priority (lower = higher priority)
mail1.example.com.: Mail server hostname

MX Priority Explained

How Priority Works

Lower number = Higher priority

example.com. MX 10 primary.mail.com.     ← Tried FIRST
example.com. MX 20 secondary.mail.com.   ← Backup
example.com. MX 30 tertiary.mail.com.    ← Last resort

Same Priority = Load Balancing

example.com. MX 10 mail1.example.com.
example.com. MX 10 mail2.example.com.

Result: Traffic split between both servers

Priority Best Practices

✅ Primary: 10
✅ Backup: 20, 30, 40, etc.
✅ Gap: Leave space (10, 20, 30) not (10, 11, 12)
❌ Don't use: Priority 0 (reserved)

Common MX Configurations

Google Workspace

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

5 servers: High redundancy

Microsoft 365

example.com. MX 0 example-com.mail.protection.outlook.com.

Single MX: Microsoft handles redundancy

Zoho Mail

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

3 servers: Primary + backups

Self-hosted (Simple)

example.com. MX 10 mail.example.com.

Single server: No redundancy

Self-hosted (Redundant)

example.com. MX 10 mail1.example.com.
example.com. MX 20 mail2.example.com.

Two servers: Primary + backup

MX Record Errors

1. No MX Records

Error: No MX records found for example.com

Problem: Can't receive email
Solution: Add MX record in DNS

2. MX Points to CNAME

example.com. MX 10 mail.example.com.
mail.example.com. CNAME hosting.example.net.

❌ Invalid: MX can't point to CNAME

✅ Fix: MX must point to A record

example.com. MX 10 mail.example.com.
mail.example.com. A 192.0.2.1

3. MX Points to IP

example.com. MX 10 192.0.2.1

❌ Invalid: MX needs hostname

✅ Fix:

example.com. MX 10 mail.example.com.
mail.example.com. A 192.0.2.1

4. Missing A Record

example.com. MX 10 mail.example.com.
# But mail.example.com has no A record

Problem: Can't resolve IP
Solution: Add A record

mail.example.com. A 192.0.2.1

5. Multiple MX, All Same Priority

example.com. MX 10 mail1.example.com.
example.com. MX 10 mail2.example.com.
example.com. MX 10 mail3.example.com.

⚠️ Warning: Load balancing only
✅ Better: Add backup priorities

Setting Up MX Records

Step 1: Choose Mail Provider

Options:

Google Workspace (business)
Microsoft 365 (business)
Zoho Mail
Self-hosted (Postfix, etc.)

Step 2: Get MX Values

Provider gives you:

Hostnames
Priorities
Optional: TXT records (SPF, DKIM)

Step 3: Add to DNS

Example (Cloudflare):

DNS → Add Record

Type: MX
Name: @
Mail server: aspmx.l.google.com
Priority: 1
TTL: Auto

Step 4: Wait for Propagation

DNS propagation: 1-48 hours

Most changes: 1-4 hours
Full global propagation: 24-48 hours

Step 5: Test

Send test email:

your-email@example.com

Check delivery:

Inbox received? ✅
Bounce? Check MX records
Spam? Check SPF/DKIM

Verifying MX Records

Method 1: MXToolbox.eu

1. Go to MX Lookup tool
2. Enter domain
3. Click "Lookup"
4. View all MX records + IP addresses

Method 2: Command Line

Mac/Linux:

dig example.com MX +short

Windows:

nslookup -type=MX example.com

Output:

10 mail.example.com.

MX Record Best Practices

Redundancy

✅ Multiple servers: Minimum 2
✅ Different priorities: Primary + backup
✅ Different locations: Geographic redundancy

Configuration

✅ Valid hostnames: Not IPs
✅ A records exist: For all MX targets
✅ No CNAMEs: MX → A record only
✅ Priority gaps: Room to insert

Security

✅ SPF record: Authorize mail servers
✅ DKIM: Sign outgoing emails
✅ DMARC: Enforce authentication
✅ TLS: Encrypted connections

Troubleshooting Email Delivery

Emails Not Arriving

Check:

MX records exist? ✅
MX records correct? ✅
A records resolve? ✅
Mail server running? ✅
Firewall allows port 25? ✅
SPF record valid? ✅

Emails Bouncing

Common causes:

No MX records
Wrong MX hostname
Mail server down
Mailbox full
Blacklisted IP

MX vs Other DNS Records

Record	Purpose
MX	Email delivery routing
A	Maps hostname → IPv4
AAAA	Maps hostname → IPv6
CNAME	Alias (not for MX!)
TXT	SPF, DKIM, DMARC

Migrating Email Providers

Safe Migration Process

Lower TTL (24h before migration)

example.com. MX 10 old-mail.com. (TTL: 300)

Add new MX (parallel)

example.com. MX 10 old-mail.com.
example.com. MX 20 new-mail.com.

Wait for propagation (1-4 hours)

Swap priorities

example.com. MX 20 old-mail.com.
example.com. MX 10 new-mail.com.

Monitor both servers
Remove old MX (after 48 hours)
```
example.com. MX 10 new-mail.com.
```

Conclusion

MX records are critical for email delivery. Proper configuration ensures:

Reliable email reception
High availability (redundancy)
Fast delivery
Professional email setup

Key points:

Use multiple MX records for redundancy
Set priorities correctly (lower = higher priority)
Point MX to A records, not CNAMEs or IPs
Test thoroughly after changes
Monitor delivery continuously

Check your MX configuration: Free MX Lookup Tool

Complete Email Setup: