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An MX record tells sending mail servers which host should receive email for a domain. When a domain lists multiple mail exchangers, the server with the lowest numerical preference is the most preferred. That number sets an order for delivery attempts; it does not measure a server’s speed or quality.
What an MX record does
MX stands for mail exchanger. An MX record associates a mail domain—such as the domain after the @ sign in a recipient address—with a hostname designated to handle incoming mail. This lets a domain use a separate mail server rather than requiring the domain name itself to accept SMTP connections. The format is defined in RFC 1035.
MX records guide incoming mail delivery. They do not configure every part of email: SPF, DKIM, and DMARC serve different purposes, including aspects of outbound authentication, and an MX record alone does not guarantee that mail will reach an inbox.
How MX preference numbers work
Each MX record contains a preference number and an exchange hostname. Sending systems sort available exchangers by preference, with the smallest number first. As RFC 5321 puts it, “Lower numbers are more preferred than higher ones.” This can feel backward in DNS control panels that label the field “priority”: a lower number usually means higher delivery preference.
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| Preference | Exchange hostname | Meaning |
|---|---|---|
| 10 | mx-primary.example. | More preferred |
| 20 | mx-secondary.example. | Less preferred; an alternate destination |
In this example, a sender normally tries the exchanger with preference 10 before the one with preference 20. The numbers establish ordering, not server capacity, quality, or speed. SMTP senders can have limits and policies for trying alternate addresses; RFC 5321 says they must be capable of trying relevant alternatives and should try at least two addresses. The standard does not guarantee identical behavior or timing from every sender.
What happens when MX records have equal preference
If multiple exchangers share the same preference, RFC 5321 says a sender should randomize among them when there is no clear reason to favor one. This can distribute delivery attempts across equivalent hosts, but it does not promise perfectly even traffic distribution.
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MX hostnames must resolve separately
An MX record points to a hostname, not directly to an IP address. After selecting an exchanger, a sender must resolve that hostname to an address before it can connect. Choosing an MX record and looking up the exchanger’s address are related but distinct DNS steps. A published MX hostname that does not resolve to usable addresses cannot serve as an effective destination.
How to check or change MX records
Use the exact hostnames and preference values supplied by the mail provider that handles the domain. The DNS standards explain how preference is interpreted; they do not prescribe provider-specific settings.
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- Find the provider’s current instructions. Use the mail service’s setup guidance for the domain’s exact MX hostnames and preference values rather than substituting example values.
- Check the authoritative DNS zone. Confirm that you are editing the zone authoritative for the domain, then look for stale or conflicting MX records before making changes.
- Verify both routing and destinations. Check that the intended MX records are published and that each exchange hostname resolves to usable addresses. Also confirm that the receiving mail service is configured to accept mail for the domain.
- Allow for DNS caching. Changes may not be visible to every sender immediately; resolvers can retain DNS data according to the record’s TTL.
Why a backup MX may not take over as expected
A higher-numbered MX record is less preferred and can provide an alternate destination, but it does not ensure immediate failover. To diagnose a delivery problem, check these points separately:
- Are the intended MX records visible in authoritative DNS?
- Has the sending system obtained current DNS data, or could it be using cached information?
- Was the preferred exchanger actually unavailable or rejecting delivery?
- Does the sender try alternate destinations, and what retry timing or limits does its implementation use?
RFC 5321 recognizes that senders can limit the alternate addresses they try. As a result, the presence of a secondary MX record alone cannot establish when—or whether—a particular sender will attempt it.
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