Definition
Email MX priority is the preference value assigned to an MX record that tells sending servers the order in which to try a domain's mail servers. A lower numeric value indicates a higher priority, so mail is delivered to the server with the lowest preference first. MX priority is what enables redundancy, allowing a backup server to accept mail when the primary is unavailable.
How It Works
A domain typically publishes several MX records, each pointing to a mail server and carrying a preference number:
example.com MX 10 mail1.example.com
example.com MX 20 mail2.example.com
example.com MX 30 mail3.example.com
When a sending server delivers mail, it queries all MX records, sorts them by preference, and attempts the lowest-numbered server first. If that server is unreachable or refuses the connection, the sender moves to the next highest number. This failover happens automatically and is the core purpose of MX priority.
Why It Matters
Correct MX priority ensures mail reaches the domain reliably and in the intended order. Common considerations include:
- The primary server should have the lowest number so it receives mail first.
- Backup servers with higher numbers provide failover during outages or maintenance.
- All servers in the set should be configured to accept mail for the domain, or mail may bounce when the primary is down.
Mistakes in MX configuration cause immediate, visible delivery failure. If the lowest-priority record points to a server that does not actually handle the domain's mail, senders may reject or bounce messages. MX priority also interacts with SPF, since some SPF records use the mx mechanism to authorise the domain's mail servers.
MX priority should not be confused with the speed of delivery. It is a routing preference, not a performance setting. The value only determines order, not how quickly mail moves.
Example
A company runs two mail servers for resilience. Its primary has an MX preference of 10 and its backup a preference of 20. During a maintenance window, the primary is taken offline, and sending servers automatically deliver to the backup. When the primary returns, mail resumes flowing to it first.
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Frequently Asked Questions
Yes. Lower preference values are tried first. A value of 10 is higher priority than 20 or 30.
At least one, but domains seeking redundancy commonly use two or more. A single MX record means there is no failover if that server goes down.
The sending server will retry for a period, then return the message as a bounce if delivery remains impossible. This is why backup MX records are important for reliability.
Yes, indirectly. Misconfigured MX records can cause mail to be rejected or bounced, damaging the sender's ability to reach the domain and, over time, the sending server's standing. Correct MX configuration is part of sound email infrastructure.