550 5.7.27: sender 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.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 550 5.7.27
- 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
550 5.7.27 Sender address has null MX
What it means
A null MX (RFC 7505) is a single MX record pointing to a dot, declaring that a domain receives no mail. Mail from such a domain cannot get bounces or replies, and an unusable return address often signals spam, so receivers may refuse it; RFC 7505 says they should use 550 5.7.27 when they do. It is permanent. RFC 7505 also advises domain owners not to publish a null MX on any domain they use as a sender, so the fix is to replace it with real MX records or to send from another domain.
Common causes
- A domain that publishes a null MX is also used in the From or envelope sender address.
- A sending subdomain set up with a null MX by mistake.
How to fix it
- 01Run an MX lookup on your sender domain.
- 02If it shows a single MX record of a bare dot, replace it with MX records for a server that receives your bounces and replies.
- 03Or send from a domain 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.
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.
- 5.1.8Sender domain cannot receive mail.The domain in the sender address does not exist or cannot accept mail, so the receiving server refuses a message whose bounces could never be returned.
- 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.
5.7.27 at other providers
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce