ė.email

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.

The MX lookup

Try

The domain you type goes to two DNS-over-HTTPS operators, Google Public DNS and Cloudflare, straight from your browser, and to nobody else. It is not stored, and it is not put in this page's address. If you type an email address, only the part after the @ is used.

MX

<domain> · MX

Not checked yet

The hosts that receive mail for the domain, their order, and whether each has an address.

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.