What Is MX Priority
Learn what MX priority means, how lower numbers indicate higher priority, backup MX, and common priority mistakes.
Introduction
When multiple MX records exist, sending servers use priority to decide which mail exchanger to contact first.
Priority does not mean “better mail service” — it only defines delivery order and failover behavior.
Quick answer
MX priority is the number in an MX record that controls try order. Lower numbers have higher priority and are tried first. MX 10 is preferred over MX 20. Equal priorities may be load-balanced.
What it means
Each MX record contains a priority value and a mail server hostname. Senders attempt the lowest priority number first, then move to higher numbers if the primary is unavailable.
- Lower number = higher priority (tried first)
- Higher number = backup or secondary MX
- Equal priorities may be used for load balancing
- MX hostname must resolve via A or AAAA
- Wrong priority can send mail to the wrong server
Where you see this:
- Primary and backup mail server setups
- Email provider migrations with temporary dual MX
- Google Workspace and Microsoft 365 redundant MX sets
- Disaster recovery mail routing
example.com MX 10 mail1.example.com
example.com MX 20 mail2.example.com
; mail1 is tried before mail2Why this matters
If an old provider still has the lowest priority, mail keeps delivering there after you thought migration finished. Backup MX that accepts but does not forward mail can cause silent delivery failures.
How to check it
- Run MX Lookup and list all MX records with priorities.
- Confirm the lowest number points to your active mail provider.
- Verify each MX hostname resolves in DNS Lookup.
- Check whether backup MX is intentional and configured to deliver.
- Send test email after priority changes.
Check MX priorities
Use MX Lookup to see priority values and mail server order for a domain.
Common mistakes
Thinking higher number means higher priority
MediumMX 20 is lower priority than MX 10, not higher.
Next step: Set active provider to the lowest priority number.
Backup MX accepts but does not deliver
HighSecondary server queues or drops mail without forwarding.
Next step: Configure backup MX to relay or remove it if unused.
Old provider still has lowest priority
HighMail continues to old infrastructure after migration.
Next step: Raise old MX priority or remove obsolete records.
MX hostname missing A record
HighPriority order does not matter if hostnames do not resolve.
Next step: Add DNS for each MX target hostname.
Example
example.com. 3600 IN MX 10 aspmx.l.google.com.
example.com. 3600 IN MX 20 alt1.aspmx.l.google.com.Frequently asked questions
What MX priority should I use?
Use values your email provider documents. Google and Microsoft publish specific priority sets. Lower numbers are primary.
Can two MX records have the same priority?
Yes. Senders may distribute load between equal-priority MX records.
Does MX priority affect sending mail?
No. Outbound SMTP uses different settings. MX priority is for incoming delivery only.
Is MX 0 valid?
Yes. Zero is the highest possible priority (lowest number).
Related tools
Use these free tools to verify your configuration after applying changes.
Related guides
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