556 5.1.10: recipient domain has a null MX
The recipient's domain publishes a null MX record, its formal statement that it never receives email, so the message was refused at once without any delivery attempt.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 556 5.1.10
- Provider
- Standard SMTP codes
- Temporary or permanent
- PermanentRefused for good: the message comes back to the sender, and resending it gets the same answer.
- Who has to act
- Either side, depending on the cause. Start with the checks below that are yours.
- What the server says
556 5.1.10 Recipient address has null MX
What it means
A domain that never receives mail can say so in DNS with a single MX record pointing to a dot (RFC 7505). When a sending or relay server finds that record for a recipient's domain, RFC 7505 says it should reject the recipient with reply code 556 and enhanced code 5.1.10. It is permanent: the domain owner has declared that nothing is delivered there, so the message is returned straight away instead of waiting in a queue. It usually means the address is wrong, for example a web host name or a misspelled domain, or that the domain is used for something other than mail.
Common causes
- The address uses a domain that is deliberately set up never to receive mail.
- A typo or a wrong domain, such as www.example.com instead of example.com.
- A domain owner who added a null MX without realising the domain was still used for mail.
How to fix it
- 01Check the domain part of the address for mistakes.
- 02Run an MX lookup on the domain: a single MX record of a bare dot with preference 0 is a null MX.
- 03Ask the recipient for an address at a domain that receives mail.
- 04If the domain is yours and should receive mail, replace the null MX with your mail server's MX records.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 556Domain does not accept mail.A relay or sending server knows the recipient's domain does not accept mail, usually from its null MX record, and refused the address without trying to deliver.
- 550 5.7.27Sender domain has a null MX.The domain in the sender address publishes a null MX record, which says it never receives mail, so the receiver refuses mail whose bounces could not be delivered.
- 5.1.2Recipient domain cannot receive mail.The part of the address after the @ names a domain that does not exist or cannot accept mail, so the message cannot be delivered there.
5.1.10 at other providers
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce