DNS or mail server lookup failed
There is a problem finding the server address for mail delivery.
What to do now
- Check the email domain for typos.
- Inspect MX and DNS answers. A Null MX declares that the domain accepts no mail.
Representative reply
Host or domain name not found: example.com
Retry decision
Retry after DNS or resolver recovery. Do not send to a Null MX domain.
Why it happens
DNS resolution may fail, or a Null MX record may declare that the domain accepts no mail.
Typical stage
CONNECT
- CONNECT
- EHLO
- MAIL FROM
- RCPT TO
- DATA
- DELIVERY
Stages vary by server. Confirm the actual stage from the complete reply and session logs.
What is SMTP Reply Code?Open only the explanations you need
Three-digit server reply: 4xx is transient, 5xx permanent. Read the complete text to investigate the cause.
What is Enhanced Status Code?Open only the explanations you need
An extended status such as 5.1.1. Server wording and usage may vary.
What is MX?Open only the explanations you need
DNS record identifying receiving mail servers for a domain.
What is Null MX?Open only the explanations you need
An explicit DNS declaration that a domain accepts no mail.
Check directly
- Check the email domain for typos.
- Inspect MX and DNS answers. A Null MX declares that the domain accepts no mail.
If you operate or diagnose the mail server, the tools below can help with these checks.
dig MX example.com openssl s_client -starttls smtp -connect mx.example.com:25 -verify_return_error -crlf EHLO test.example MAIL FROM:<sender@example.com> RCPT TO:<user@example.com>
Cautions
Observed DSNs do not map one-to-one to causes. The stage shown is typical; confirm it with the full reply and actual logs.
Similar reply wording and observed status codes
host or domain name not foundunable to look up hostname or service not knownno address associated with hostnamenullmxname service error
Observed DSNs: 5.4.4, 5.1.2, 4.4.3, 4.4.4