An order, not a score

Suppose an example domain publishes the two illustrative records below. A sender normally starts with mail-a.example.org. The second route may be tried when delivery to a preferred destination is not possible, subject to the sending server’s retry behavior.

10  mail-a.example.org20  mail-b.example.org

The distance between the numbers is irrelevant: 10 and 100 establish an order, not a 10× difference in performance or capacity.

When the numbers match

Hosts with the same preference are peers in the delivery order. Senders can vary which one they try first. Our results sort equal-priority names alphabetically so the display stays easy to compare; that alphabetical order is not a DNS instruction.

The “Preferred” label in this tool appears on every ordinary record with the lowest returned preference. It does not represent a connectivity test or a recommendation to delete other records.

Do not invent a backup route

A backup mail server needs to be configured to accept and forward mail for your domain. Pointing a second MX at an unrelated server will not create redundancy. Follow the exact set of records supplied by your mail provider, and remove obsolete routes only after checking that they no longer serve your mail setup.

A practical migration comparison

  1. Save the current lookup before the migration.
  2. Check the new provider’s required hostname and priority together.
  3. Make the authorized change at the authoritative DNS provider.
  4. Compare Cloudflare and Google after the old TTL has had time to expire.
  5. Run an actual end-to-end test email and inspect the receiving mailbox.

Matching public DNS answers support the DNS part of the check. They do not replace the final delivery test.

Zero has two distinct uses

A normal hostname with priority 0 is simply preferred over a hostname with priority 10. The special combination 0 . means no incoming mail service. This tool treats that as Null MX, not as a high-priority working mail server. A Null MX combined with other MX records is flagged as a conflicting configuration.

References & further reading

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