5.1.8: sender 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.
- Standard
- Provider
- Permanent (5xx)
- Class
- Sources checked
- Author
At a glance
- Code
- 5.1.8
- 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
5.1.8 Bad sender's system address
What it means
Many receivers check that the envelope sender's domain is real before accepting a message: it must exist in DNS and be able to receive mail, because that is where any bounce would go. 5.1.8 reports that this check failed. RFC 3463 ties it to no class. A 5.1.8 is a refusal and the message comes back. A 4.1.8 asks the sender to try again later, which servers often do when the sender domain could not be resolved at that moment; if the domain really does not exist, every retry fails until the message expires. An unregistered or misspelled domain, or a subdomain without DNS records of its own, is the usual trigger.
Common causes
- A sender domain that is misspelled, expired or never registered.
- A subdomain used in the return path that has no DNS records of its own.
- A temporary DNS failure while the receiver looked the domain up.
How to fix it
- 01Check that the domain in your MAIL FROM address resolves, for example with an MX lookup.
- 02If you send from a subdomain such as bounce.example.com, give it MX or address records so it can receive bounces.
- 03Renew the domain if it has lapsed.
- 04If the domain resolves fine and the code was 4.1.8, let the queue retry.
Sources
What this page rests on, checked on 20 Oct 2026. The explanation is ours; the reply text is the server's.
Related codes
- 5.1.7Sender address is malformed.The sender address in the envelope (MAIL FROM) is written in a way the receiving server cannot accept, so it refused the message.
- 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.
- 4.4.3DNS or directory lookup failed.The message could not be passed on because a directory server, most often DNS, did not answer; the lookup and the delivery are retried.
5.1.8 at other providers
Further reading
All 105 standard codesEvery code in the wikiPaste your own bounce