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SenderProbe
Routing check

Check the MX records a domain publishes

MX records are the first thing a sending server asks DNS for. SenderProbe reads every mail exchanger a domain publishes, resolves the hosts behind them, opens a connection to each one and reports which part of the routing is missing.

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What an MX record is

An MX record names a host that accepts mail for a domain, together with a preference value that says in which order senders should try it. It is published at the domain itself: mail addressed to a mailbox at example.com is routed by the MX records at example.com, not by anything under it.

The target of the record is a hostname, never an address. RFC 2181 rules out an MX target that is a CNAME, and an address literal is not a hostname at all, so a receiver that enforces either treats the record as a misconfiguration rather than guessing what was meant.

Two mail exchangers for example.com, each with an address record of its own. Senders try preference 10 first and fall back to 20.
example.com.        3600  IN  MX    10 mx1.example.net.
example.com.        3600  IN  MX    20 mx2.example.net.
mx1.example.net.    3600  IN  A     203.0.113.10
mx2.example.net.    3600  IN  A     203.0.113.11

What the MX lookup checks

An MX record check that stops at the record set only tells you what DNS says. SenderProbe resolves every hostname, opens an SMTP connection to each mail host and forward-confirms the reverse DNS of the addresses behind them, because a published mail exchanger that nobody can reach is worth the same as no record at all.

  • Every MX hostname, with its MX preference value and TTL.
  • The A and AAAA records behind each hostname, and any host that publishes none.
  • Whether each mail host answers on port 25, and how long it took to answer.
  • Forward-confirmed reverse DNS for the addresses those hosts resolve to.
  • A null MX record, which declares that the domain receives no mail.
  • The mailbox provider the hostnames identify, when they are recognisable.

Why mail bounces when the MX records are wrong

A domain with no MX record is not undeliverable by default, which is what makes the failure confusing. RFC 5321 tells a sender that finds no MX to fall back to the domain's A or AAAA record, so the mail is offered to whatever host answers there — usually a web server, which refuses it. The bounce then blames that host rather than the missing record.

A single mail exchanger is a single point of failure. When it stops answering, senders queue and retry for a few days and then give the message up. Two hosts on separate networks is the smallest arrangement that survives one of them failing, and it costs nothing but a second record.

Preference values and the null MX record

The number before the hostname is a preference, not a priority or a weight. A sender tries the lowest number first and moves up only when it cannot connect, so equal numbers are the only way to spread traffic across two hosts. Nothing in SMTP divides mail in proportion to the values.

A domain that sends mail but never receives any should say so. A null MX is one record with preference 0 and a target of a single dot, and it tells senders to bounce at once instead of retrying against a host that will never accept.

A null MX record as defined by RFC 7505: example.org accepts no mail, and a sender should fail immediately rather than queue for days.
example.org.  3600  IN  MX    0 .

MX records and the checks that follow them

Routing is the first of four gates a message passes. Once mail reaches the right host, the receiver still has to authenticate it, weigh the reputation of the address it arrived from and finish an SMTP conversation with it. A domain can pass this check and still have every message filtered.

The MX hostnames also tell you whose instructions to follow. A domain whose mail is hosted by a provider almost always needs that provider's SPF include and its DKIM selector rather than records of its own, and a mail server lookup that names the provider is the quickest way to find out which.

Run this check on your own domain. A workspace takes a minute to create and runs every one of the eleven checks against the domain you name.

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Common questions

What does an MX lookup show?
An MX lookup returns the mail exchanger records a domain publishes: the hostname of each mail server and the preference value that decides the order senders try them. SenderProbe adds what the record alone cannot tell you, resolving each hostname to its addresses, opening a connection on port 25 and checking whether reverse DNS is forward-confirmed.
Does a domain need an MX record to receive email?
In practice, yes. RFC 5321 lets a sender fall back to the domain's A or AAAA record when no MX record exists, so mail is offered to whatever host the domain resolves to, which is usually a web server that refuses it. Publishing an MX record is the only reliable way to say where mail should be delivered.
What is a null MX record?
A null MX is a single record with preference 0 whose target is a lone dot. RFC 7505 defines it to declare that a domain accepts no mail at all, so senders bounce immediately rather than retrying for several days. It is the correct record for a domain you send from but never receive on.
How many MX records should a domain have?
At least two, on hosts that do not share a network or a power supply. A sender tries the lowest MX preference first and moves to the next only when the connection fails, so the second host turns an outage into a delay. Every host in the list has to be able to accept mail for the domain.
Can an MX record point at an IP address?
No. The target of an MX record must be a hostname that has an address record of its own, and RFC 2181 also rules out pointing it at a CNAME. Receivers that enforce this treat an address literal or a CNAME target as a broken configuration, and some refuse the delivery rather than work around it.

Keep reading

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