MX · RFC 5321

MX lookup: which hosts receive a domain's mail

Type a domain to see the hosts that take its mail, in the order senders try them, and whether each host has an address. A missing MX and a null MX are explained, not just reported.

Check a domain All seven records at once The email doctor grades them

What an MX record is

An MX record names a host that accepts mail for a domain, with a preference number. A sending server looks up the MX records of the domain after the @, sorts them, and tries the hosts in order: "lower numbers are more preferred than higher ones" (RFC 5321 §5.1). When several hosts share a number the sender picks among them at random, which spreads the load.

example.com.  MX  10 mx.example.com.
example.com.  MX  20 mx2.example.com.

Two mail hosts, the first preferred. The zone of the example in RFC 7208 §10.1.1.

How to read the result

A list of hosts. The checker shows each with its preference and looks up its addresses, for up to 10 hosts. The host named in an MX record must have at least one address record, A or AAAA (RFC 5321 §5.1).
No MX record. Mail is not necessarily lost. If the list is empty the sender treats the domain itself as the mail host, with preference 0: the implicit MX (RFC 5321 §5.1). The checker says whether the name has an address to fall back to.
A null MX. A single record with preference 0 and a dot for the host states that the domain accepts no mail. Senders can then report the failure at once instead of retrying (RFC 7505 §3).
No such name. The domain does not exist in the DNS, and mail to it is an error.

The common mistakes

An MX that points at an alias. The name in an MX record "must not be an alias": it has to own its address records, never a CNAME (RFC 2181 §10.3).
An MX whose host has no address. A host removed from the DNS and still listed makes senders wait and retry.
A null MX beside real ones. A domain that publishes a null MX must publish no other MX record (RFC 7505 §3).
A null MX on a domain that sends mail. Receivers may reject mail whose return address cannot receive replies (RFC 7505 §4.2).
An IP address in place of a host name. The data of an MX record must be a domain name (RFC 5321 §5.1).

What this lookup does not tell you

It reads the DNS. It does not connect to the mail hosts, so it cannot say whether they are up, whether they accept mail for the domain, or whether they offer TLS. A domain that wants senders to insist on TLS says so with MTA-STS. For a domain that sends no mail at all, a null MX pairs with the SPF record v=spf1 -all (RFC 7505 §4.2); the SPF checker reads that one.

Method, sources and privacy

How this page reads the record.

Where the answers come from

Each question goes from your browser to two public DNS-over-HTTPS resolvers: Google Public DNS, at https://dns.google/resolve, and Cloudflare, at https://cloudflare-dns.com/dns-query, asked with Accept: application/dns-json. Both answer live, at the moment you ask, from their own caches: a record changed a moment ago can still be served in its old form until its time to live runs out (RFC 2181 §8). The answers are compared. When they differ both are shown; when one does not answer within 8 seconds the page names it; when neither answers, the card says so and concludes nothing. The panel This run dates the reading to the second, in UTC.

What is computed here

Everything else happens in your browser: the hosts are put in order of preference and up to 10 of them are looked up for an IPv4 and an IPv6 address; a name with no MX is checked for the address a sender falls back to. Every finding links to the section of the document it rests on. The documents this page was written against are listed here, each with the date it was read; all seventeen are on the method page.

What happens to what you type

The domain goes to those two resolvers and to no one else: not to labs.llc's server, not into the page address, not into your browser's storage, and not to analytics, whose tag records this page's address, which never holds it. Of an email address only the part after the @ is used; the rest is dropped in your browser. The two resolvers see the question and the Origin header of labs.llc, and handle it under their own policies (Google Public DNS, Cloudflare). Changing the edition, paper or dark, reloads the page as it does everywhere on labs.llc, and clears the reading on screen: nothing of it is kept to bring back.