SMTP error 5.1.10 Recipient address has null MX

What SMTP error 5.1.10 means

The domain you are writing to publishes a null MX, a single mail record at priority 0 pointing at a bare dot, which is a formal declaration that it accepts no email at all. There is no server to attempt, so the message is refused immediately as 556 5.1.10 with no queueing and no retries. The address is not mistyped in any ordinary sense. The domain simply does not do email.

Address status
Sent by Your own outbound mail server, or the first relay that looked the recipient domain up
Occasional

What SMTP error 5.1.10 Recipient address has null MX actually means

Look at where this one comes from, because it is unlike most of the family: you will normally see it generated by your own mail system rather than by a distant one, since the lookup fails before any connection is attempted. That is why the bounce lands within seconds instead of after the usual retry window, and why the reply code is 556, reserved for exactly this condition. Encountering it almost always means the address is out of date or subtly wrong, most often a company's marketing or web domain used in place of its actual mail domain. It is the mirror image of 5.7.27, where it is the sending domain that has declared itself mail-free.

This status code is returned when the associated address is marked as invalid using a null MX.
— RFC 7505 with RFC 7504

How 5.1.10 appears in a bounce

A server reporting this condition sends it alongside a three-digit reply code, and a bounce prints the two together. The registry lists 556 5.1.10 as the pairing for this status.

The leading 5 is the server's verdict rather than part of the code's identity: it marks this as a permanent failure, which means the sending server has given up and returned the message.

What causes SMTP error 5.1.10 on a business phone system

A brand or web domain used as if it were the mail domainGroups often trade under one name and run email under another. Someone reads the address off the website, assumes the two match, and sends to a domain that has never been set up to receive.
A business that has wound down cleanlyPublishing a null MX is the tidy way to close a domain, because it tells senders immediately rather than leaving them retrying for four days. Old contacts in your CRM will find it long before anybody tells you the company closed.
A parked or registrar-held domainA typo can land on a domain that genuinely exists but is held for resale, and registrars increasingly publish a null MX on parked names rather than accepting mail into a void. Check the spelling before assuming anything else.
A domain caught mid-migrationSome DNS templates apply a null MX as a placeholder when records are cleared down. If a normally reliable correspondent starts failing this way, they have probably broken their own DNS and would want to be told.
Contact details harvested from a websiteAddresses scraped or copied from a site footer sometimes belong to a campaign domain that only ever hosted a landing page. The domain resolves, the site loads, and no mail was ever configured for it.

How to fix SMTP error 5.1.10

Check the address against a message they sent youThe most reliable source of a working address is an email the person has already sent. Compare the domain in that message with the one you are using, since a one-word difference is easy to miss.
Look up the MX records for the domain yourselfA query returning a single record with preference 0 and a lone dot confirms the diagnosis outright. That takes a few seconds and saves an argument about whether your mail system is at fault.
Telephone the recipientYou cannot email someone whose domain accepts no email, so the loop has to be closed another way. If they are a live customer, tell them what their DNS says, because plenty of businesses do not know.
Clean the record out of your CRM with confidenceUnlike a soft bounce or a full mailbox, this result is definitive and will not change tomorrow. Addresses on null MX domains can be removed or flagged permanently rather than left to fail on every future campaign.
Do not put the domain on a retry scheduleThere is no server to try again later, so queueing achieves nothing but delay. If your application retries these automatically, teach it to treat 556 as final.

Fixing the underlying problem

This page explains the code. These guides walk through the fix in detail.

Questions about SMTP error 5.1.10

Why did this bounce arrive so much faster than a normal one?

Because no connection was ever made. Your server read the recipient domain's DNS, saw a declaration that it accepts no mail, and stopped there. Ordinary failures need a conversation with a remote server first, which is why they take longer.

Could their domain be broken rather than deliberately closed?

It is possible. A null MX is rarely created by accident, but DNS templates and migration scripts can apply one, and the owner may have no idea. If it is a customer or supplier, ring them rather than assuming they have shut down.

Is 556 a genuine SMTP reply code?

Yes. It was added specifically for this situation so that a server could distinguish a domain that refuses all mail from an address that merely does not exist. Older systems that predate it sometimes report the same condition as 5.1.2 instead.

Related SMTP status codes

Source. SMTP error 5.1.10 Recipient address has null MX is defined in RFC 7505 with RFC 7504 and registered in the IANA SMTP Enhanced Status Codes registry. The causes and fixes above are drawn from our own experience supporting UK business email systems.

All Email delivery & SMTP codes