MX lookup
See which servers receive email for a domain, in the order senders try them, with each server's addresses.
A domain, an email address or a URL. Lookups go from your browser to Cloudflare's public DNS (1.1.1.1); nothing is sent to simmail.
What an MX record is
An MX (mail exchanger) record tells the rest of the internet which servers accept email for a domain. When someone sends a message to you@example.com, their mail server looks up the MX records for example.com, sorts them by preference (lowest first) and connects to the first one that answers on port 25.
Each record has two parts: the preference number and a host name, for example 10 mx1.example.com. The host name then needs its own A (IPv4) or AAAA (IPv6) records, which is why this tool resolves every host as well.
Why it matters
If MX records are missing or wrong, mail to the domain is delayed or bounced. Senders retry for a while (often several days) when no server answers, so a broken MX can go unnoticed until people ask why nobody replied. MX records also feed into other checks: the mx mechanism in SPF and the mx: patterns in an MTA-STS policy both depend on them.
Common mistakes
- MX pointing at an IP address. An MX must hold a host name. Create an A record such as
mx.example.comand point the MX at that. - MX pointing at a CNAME. RFC 2181 and RFC 5321 forbid aliases here. Point the MX straight at the final host name.
- Hosts with no address. A typo in the host name, or a server that was retired, leaves senders with nowhere to connect.
- No MX at all. Senders fall back to the domain's own A record. If no mail server runs there, messages bounce after retries. A domain that never receives mail should publish a null MX (
0 ., RFC 7505) so senders fail fast. - Leftover records after a migration. Old provider MX records with a low preference keep pulling mail to the old system. Remove them once the move is done.
Receiving mail is only half of it. If the domain also sends mail, check its SPF, DKIM and DMARC records too: since February 2024 Gmail and Yahoo require SPF or DKIM from every sender, and SPF, DKIM and DMARC from bulk senders.
Questions
What does the number before each MX host mean?
It is the preference. Senders try the lowest number first and move to higher numbers only if they cannot connect. Hosts with the same number share the load.
What happens if a domain has no MX record?
RFC 5321 says senders then treat the domain itself as the mail server and deliver to its A or AAAA address. That works only if a mail server listens there, so publish MX records if the domain receives mail.
What is a null MX?
A single MX record with preference 0 and the host "." (RFC 7505). It says the domain never accepts email, so senders bounce messages at once instead of retrying for days.
Can an MX record point to an IP address or a CNAME?
No. An MX must name a host that has its own A or AAAA records. An IP address in an MX is invalid, and an alias (CNAME) is forbidden by RFC 2181 and RFC 5321, though many senders tolerate it.
Is this lookup cached?
Answers come from Cloudflare's public resolver, which caches records for their TTL. A change you just made can take until the old TTL runs out to show up.
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