MX record priority explained: lower numbers, failover, and equal values

example.com. MX 10 mx-primary.example.net.
example.com. MX 20 mx-secondary.example.net.
example.com. MX 30 mx-last-resort.example.net.

Values do not need to be consecutive. 1, 5, 100 behaves the same as 10, 20, 30 because only ordering matters. Leaving gaps is convenient when a provider later asks you to insert a destination between existing levels.

Hosts with the same preference are equally preferred. A sending server should randomize or otherwise spread attempts among them rather than always using the first DNS answer. Providers use equal values for distribution across multiple inbound clusters.

example.com. MX 10 mx1.provider.net.
example.com. MX 10 mx2.provider.net.

When the most-preferred destination is unreachable, a sender can try a less-preferred host. Temporary SMTP errors may instead cause queueing and later retries, depending on the failure and the sender's implementation. MX preference is destination selection, not a promise that every sender changes hosts immediately.

A secondary host also needs the correct recipient directory, anti-spam policy, TLS, and relay behavior. An outdated backup that accepts messages for nonexistent users or forwards without equivalent controls can create backscatter and security problems.

Attackers may deliberately target the less-preferred server, hoping it has weaker filtering than the primary. If the backup accepts mail broadly and later relays it, it becomes an easier path into the same mailboxes. Modern hosted providers usually supply all required MX destinations as one managed system; adding an unrelated backup is rarely helpful.

• Use only provider-approved destinations.
• Apply the same recipient validation and anti-abuse controls everywhere.
• Do not configure an open relay.
• Monitor all MX hosts for reachability and certificate problems.

1. Record the current public MX set and its TTL.
2. Obtain the complete new record set from the receiving provider.
3. Publish its hostnames and values exactly, removing old-provider entries when instructed.
4. Run the MX lookup and verify the lowest values belong to the intended preferred destinations.
5. Send mail from an unrelated external provider and confirm real delivery.
6. Continue checking during the old TTL window and watch both old and new systems during a migration.

If you are new to inbound routing, start with what is an MX record. Remember to review SPF, DKIM, and DMARC separately whenever an email-provider migration also changes outbound sending.