Standard SMTP codes · permanent error
556: domain 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.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 556
- 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
- The sender. The fix is on the sending side: your domain, your server or your list.
- What the server says
556 Domain does not accept mail
What it means
RFC 7504 added 556 for an intermediate server that can tell, without connecting, that the destination does not take mail. The standard way to tell is a null MX record (RFC 7505), and RFC 7505 pairs 556 with enhanced code 5.1.10. The client treats it like any permanent negative reply, so the message is returned at once instead of waiting in a queue. It nearly always means the address has the wrong domain.
Common causes
- The recipient domain publishes a null MX record.
- A typo or a wrong domain that happens to publish a null MX.
How to fix it
- 01Check the domain in the address.
- 02Run an MX lookup to confirm the domain publishes a null MX.
- 03Ask the recipient for an address that receives mail.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
- RFC 7504, section 4: The 556 Reply Coderfc-editor.org
- RFC 7504, section 5.1: Specific Changes to RFC 5321rfc-editor.org
- RFC 7505, section 4.1: SMTP Server Benefits (Null MX)rfc-editor.org
Related codes
- 556 5.1.10Recipient 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.
- 521Host does not accept mail.The server answered the connection only to say that it never accepts mail, so delivery to this host cannot work.
- 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.
556 at other providers
- 556Recipient domain publishes a null MX.The domain you are writing to publishes a null MX record, which declares that it accepts no email, so the message cannot be delivered.
- 556Recipient domain publishes a null MX.The domain you are writing to has a No Service MX record (null MX), which means it can neither receive nor send email, so the message cannot be delivered.
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce