556 5.1.10: The recipient domain publishes a null MX: it takes no mail

At a glance

• Type: permanent failure (class 5, RFC 3463 §2). The sending server does not retry; sending the same message again fails the same way until the cause is fixed.
• Basic code 556: “Domain does not accept mail” (RFC 7504 §4).
• Enhanced code 5.1.10: “Recipient address has null MX” in the IANA registry (RFC 7505), in the group addressing status (RFC 3463 §3.2).

The recipient's domain has said, in DNS, that it never accepts email. Your message was refused before any delivery was attempted, and no address at that domain will work.

What it means

RFC 7505 §3 defines the null MX: a domain that accepts no mail publishes exactly one MX record with preference 0 and a target of . (a single dot, meaning no host). The dot cannot be a real host name, so the record cannot be mistaken for a normal MX. A domain with a null MX must not publish any other MX record.

When a sending or relay server sees that record while handling a recipient, RFC 7505 §4.1 says it should reject the recipient with basic code 556 and enhanced code 5.1.10. RFC 7504 §4 added 556, "Domain does not accept mail", for exactly this situation, and says the client should treat it like any other permanent failure. The benefit, per RFC 7505 §4.1, is that the sender learns at once instead of retrying for days.

Do not confuse this with Microsoft's 550 5.1.10, which means a mailbox was not found.

Common causes

• The address uses a domain that only serves a website and has opted out of email, such as a www. subdomain.
• A typo turned the intended domain into a different, real domain that publishes a null MX.
• A company parked or retired a domain and published a null MX to stop mail to it.

How to check it

Look up the domain's MX records, for example with dig MX example.com. A null MX shows as a single record like 0 .. If you see that, the bounce is correct and expected.

How to fix it

1. Look for a typo in the domain. RFC 7505 §4.1 itself gives mistyped addresses such as a www. prefix or a wrong top-level domain as cases null MX is meant to catch.
2. Ask the person for an address at a domain that receives mail.
3. If the domain is yours and you do want to receive mail, remove the null MX and publish MX records for your mail service.
4. If the domain is yours and you also send from it, note that RFC 7505 §4.2 says domains used as sender addresses should not publish a null MX; receivers may reject such mail with 550 5.7.27.

What to do with the address

Remove it. The domain owner has stated that no mail is accepted, so every address at that domain will fail the same way.

Related codes

• 553 5.1.2: The recipient domain does not exist or takes no mail
• 550 5.1.10: Recipient not found (Microsoft 365)
• 550 5.7.27: SPF failed for a bulk sender (Gmail), or the sender has a null MX

See every bounce code in the dictionary.

Many bounces from addresses that do not exist can be avoided: check an address with our free email checker, or clean a whole list before you send.

How we wrote this: drafted by the email-validation.org team with the help of AI writing tools, from our product documentation and the sources linked in the text.