556 5.1.10: recipient address has null MX
The recipient's domain says, in DNS, that it accepts no email at all (a null MX record). The message can't be delivered to any address at that domain.
What 556 5.1.10 means
RFC 7505 defines the "null MX": a single MX record with preference 0 and a host of "." that a domain publishes to say it doesn't accept email. A domain with a null MX must not publish any other MX record.
When a sending or relay server finds a null MX for the recipient's domain, RFC 7505 says it should reject the recipient with reply code 556 and enhanced code 5.1.10, "Recipient address has null MX". The 556 reply code itself comes from RFC 7504: an intermediate system that can tell a destination won't accept mail, without trying to connect, returns 556. The IANA registry lists X.1.10 with 556 as its basic code.
Often this comes from your own provider's servers, before your message leaves. It's a clear answer, not an error to work around: the domain owner has deliberately said it doesn't take mail. The address is probably a typo, an old company domain, or a web-only domain. Microsoft 365 also uses the number 5.1.10 for a different reason, "Recipient not found"; see the Microsoft entry for that.
Likely causes
- The domain doesn't use email and publishes a null MX to say so.
- A typo turned the address into a different, mail-less domain.
- The organization moved to another domain and retired email on the old one.
How to fix it
If you sent the message
- Check the domain spelling and ask the recipient for their current address.
- Remove the address from mailing lists; it will never accept mail.
If you're the recipient or their admin
- If your domain should receive email, replace the null MX with MX records for your mail provider or filtering service.
Frequently asked questions
What does a null MX record look like?
It is a single MX record with preference 0 and a host of "." (a dot), defined in RFC 7505. An MX lookup on the domain shows it. A domain with a null MX must not publish any other MX record.
Should I publish a null MX on my unused domains?
If a domain never receives email, yes: senders then fail immediately with a clear reason. Pair it with SPF "v=spf1 -all" and a DMARC reject policy so the domain can't be used for spoofing either.
Sources
- RFC 7505: A "Null MX" No Service Resource Record for Domains That Accept No Mail
- RFC 7504: SMTP 521 and 556 Reply Codes
- IANA: SMTP Enhanced Status Codes registry
Provider messages are quoted as the provider publishes them. Servers can change their wording, so the bounce you received may differ slightly.
Related codes
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