What is an MX Record?
An MX (Mail Exchange) record is a DNS record type that specifies which mail servers are responsible for receiving email on behalf of a domain. When someone sends email to user@example.com, the sending mail server looks up MX records for example.com to find where to deliver the message.
How MX Records Work
The email delivery process:
1. Sender composes email to user@example.com
2. Sender's server queries DNS for example.com MX records
3. DNS returns one or more MX records with priorities
4. Sender connects to the highest-priority mail server
5. If unavailable, sender tries the next priority server
6. Email delivered or queued for retry
MX Record Format
example.com. IN MX 10 mail.example.com.
example.com. IN MX 20 mail-backup.example.com.
Components:
- example.com. - Domain receiving email
- IN MX - Record type
- 10 - Priority (lower = higher priority)
- mail.example.com. - Mail server hostname
Priority Numbers
Priority determines the order servers are tried:
- 10 - Primary mail server (tried first)
- 20 - Secondary (fallback if primary fails)
- 30 - Tertiary (last resort)
Lower numbers = higher priority. Many setups use increments of 10 to allow inserting servers later.
Common MX Configurations
Self-Hosted Email
@ IN MX 10 mail.example.com.
mail IN A 203.0.113.50
Google Workspace
@ IN MX 1 aspmx.l.google.com.
@ IN MX 5 alt1.aspmx.l.google.com.
@ IN MX 5 alt2.aspmx.l.google.com.
@ IN MX 10 alt3.aspmx.l.google.com.
@ IN MX 10 alt4.aspmx.l.google.com.
Microsoft 365
@ IN MX 0 example-com.mail.protection.outlook.com.Zoho Mail
@ IN MX 10 mx.zoho.com.
@ IN MX 20 mx2.zoho.com.
@ IN MX 50 mx3.zoho.com.
MX Record Requirements
Target Must Be a Hostname
MX records must point to hostnames, not IP addresses:
# Correct
@ IN MX 10 mail.example.com.
# Incorrect - won't work
@ IN MX 10 203.0.113.50
Target Needs A/AAAA Record
The mail server hostname must resolve to an IP:
@ IN MX 10 mail.example.com.
mail IN A 203.0.113.50
No CNAME Targets
MX records should not point to CNAME records (RFC 2181):
# Avoid this
mail IN CNAME host.provider.com.
@ IN MX 10 mail.example.com.
Checking MX Records
Using dig:dig example.com MX
; ANSWER SECTION:
example.com. 300 IN MX 10 mail.example.com.
example.com. 300 IN MX 20 mail-backup.example.com.
curl "https://domscan.net/v1/health?domain=example.com"
# Returns hasMX: true/false in DNS details
MX Records and Email Security
MX records work alongside other email authentication records:
| Record | Purpose |
|---|---|
| MX | Route incoming email |
| SPF | Authorize sending servers |
| DKIM | Sign outgoing messages |
| DMARC | Policy enforcement |
All four should be configured for proper email operation and deliverability.
Troubleshooting MX Issues
No MX Records
If no MX records exist, some servers fall back to A record lookup—but this is unreliable. Always configure explicit MX records.
Wrong Priority
If a backup server has lower priority (higher number) than intended, it won't receive email unless the primary fails.
Hostname Resolution Failure
If the MX target hostname doesn't resolve, email delivery fails. Verify A records exist for mail server hostnames.
TTL Considerations
When migrating email providers, lower MX TTLs beforehand to speed up propagation, then restore normal TTLs after migration.
Best Practices
1. Always have MX records: Don't rely on A record fallback
2. Configure backup servers: Use multiple MX records with different priorities
3. Verify target resolution: Ensure mail server hostnames have A records
4. Implement email authentication: Add SPF, DKIM, and DMARC alongside MX
5. Monitor email delivery: Use tools to test MX configuration