Error map
Postfix: Host or domain name not found Postfix: status=deferred (Host or domain name not found. Name service error for name=example.com type=A: Host not found, try again)
Email / SMTP · layer 2, Network
Centre: this error. First ring: errors it is linked with (12). Outer ring: what those are linked with. Colours show the layer of a request each error belongs to. Click any dot to re-centre the map on it; hover to follow its connections. The centre dot opens the error page.
Directly related (12)
SMTP: 550-5.7.25 The IP address sending this message does not have a PTR record setup, or the corresponding forward DNS entry does not point to the sending IP
Gmail (and others) require your sending IP to have a reverse DNS (PTR) record that resolves back to the same IP; yours has none o…
Postfix: status=deferred (connect to gmail-smtp-in.l.google.com[142.250.27.26]:25: Connection timed out)
Postfix could not reach the remote mail server on port 25, so the message stays in the queue and is retried.
Postfix: warning: SASL authentication failure: cannot connect to saslauthd server: No such file or directory
Postfix could not authenticate a user (inbound) or could not log in to a relay (outbound) because the SASL service or credentials…
Networking: dig: ;; ->>HEADER<<- opcode: QUERY, status: SERVFAIL — resolver failed to answer (DNSSEC validation or upstream failure)
The DNS resolver couldn't get a valid answer, often because DNSSEC signatures are broken or the authoritative servers fail.
DKIM: dkim=permerror (no key for signature) / key not found in DNS selector._domainkey
The message is signed with a selector, but the receiver found no DKIM public key in DNS at that selector, so verification cannot…
Postfix: fatal: bind 0.0.0.0 port 25: Address already in use
Postfix cannot start its SMTP listener because another process (often Sendmail, Exim or a second Postfix) already uses port 25.
DMARC: dmarc=none (no DMARC record found for example.com) — bulk sender requirements not met
The sender's domain publishes no `_dmarc` TXT record, so receivers cannot apply any policy; Gmail and Yahoo now require one for b…
SMTP: 550 5.1.2 Domain not found / 554 5.4.4 Unable to resolve MX / Host or domain name not found
Mail cannot be routed because the recipient domain has no MX (or A) record, or DNS for it fails.
SMTP: telnet smtp.example.com 25 — Connection timed out (cloud provider blocks port 25)
Connections from your server to other mail servers on port 25 time out because your cloud host or ISP blocks outbound SMTP to fig…
SMTP: 504 5.5.2 <localhost>: Helo command rejected: need fully-qualified hostname
The receiving server refused your EHLO/HELO greeting because the name you sent is not a valid fully-qualified domain name.
Networking: DNS changed but the domain still resolves to the old IP — TTL and resolver caching ("propagation")
You updated a DNS record, but resolvers and devices keep serving the old answer until its TTL expires.
Networking: dig: status: REFUSED / recursion requested but not available — the DNS server won't resolve queries for you
The DNS server answered, but its policy refuses to look up names for your address.
Two steps away (30)
cURL error 6: Could not resolve host: api.example.com
PHP cURL couldn’t resolve the hostname via DNS.
sudo: unable to resolve host <hostname>: Name or service not known
The machine’s hostname isn’t in /etc/hosts (or DNS), so sudo prints this warning — usually after changing the hostname.
ping: google.com: Temporary failure in name resolution
The Linux host can’t resolve domain names — no working DNS server is configured or reachable.
SMTP: 421 4.7.0 Our system has detected an unusual rate of unsolicited mail originating from your IP address. Mail sent from your IP address has been temporarily rate limited
The receiving server (often Gmail or Outlook) is temporarily throttling your mail because the volume or reputation of your sendin…
SMTP: 450 4.7.1 <user@example.com>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/example.com.html
The recipient server deliberately rejected the first delivery attempt from an unknown sender (greylisting) and expects a legitima…
SMTP: 451 4.3.0 Error: queue file write error / 451 Requested action aborted: local error in processing
The server hit an internal problem (disk, database, content filter, DNS) while handling your message and asked you to retry later.
SMTP: 530 5.7.0 Must issue a STARTTLS command first / 530 5.7.1 Authentication required
The server will not accept mail until you encrypt the connection (STARTTLS) and/or authenticate, and your client did neither.
SMTP: 534-5.7.9 Application-specific password required. Learn more at https://support.google.com/accounts/answer/185833
The Google account has 2-Step Verification on, so SMTP clients cannot use the normal password; they need an app password (or OAut…
SMTP: 535-5.7.8 Username and Password not accepted. Learn more at https://support.google.com/mail/?p=BadCredentials
The server received your login but rejected it — wrong username or password, an unusable password type, or a blocked sign-in.
ACM certificate stuck in Pending validation
ACM is waiting to see the DNS (or email) validation record — it wasn’t created, was created in the wrong DNS zone, or has a typo.
dial tcp: lookup registry-1.docker.io: no such host / Temporary failure in name resolution during docker build
DNS isn’t working for the Docker daemon or inside build containers, so pulls and package downloads fail.
DNS not working inside a Kubernetes pod (could not resolve host / nslookup: can't resolve)
Pods can’t resolve service names or external hosts — CoreDNS is down, blocked by NetworkPolicy, or the upstream resolver fails.
550 5.7.26 This mail has been blocked because the sender is unauthenticated (SPF/DKIM/DMARC)
Gmail/Outlook rejected the message because SPF and DKIM didn’t pass for the sender’s domain (DMARC failure).
SPF: spf=permerror — Too many DNS lookups (limit is 10)
Your SPF record needs more than 10 DNS lookups to evaluate, so receivers treat SPF as a permanent error and the check fails.
SPF: spf=permerror — Multiple SPF records found for example.com
The domain publishes more than one TXT record starting with `v=spf1`; receivers must treat that as a permanent error.
SPF: Received-SPF: softfail (google.com: domain of transitioning bounce@example.com does not designate 203.0.113.5 as permitted sender)
The server that sent the mail is not listed in the sender domain's SPF record, so the receiver marks it fail or softfail.
DMARC: dmarc=fail (p=REJECT) — SPF and DKIM pass but domains are not aligned with the From header
Authentication passed for another domain (your ESP's), but neither the SPF Return-Path domain nor the DKIM `d=` domain matches th…
550 5.1.1 The email account that you tried to reach does not exist (User unknown)
The recipient mailbox doesn’t exist on the receiving server — a typo in the address, a deleted account, or a wrong domain.
SMTP: 554 5.4.14 Hop count exceeded - possible mail loop
The message passed through too many mail servers (usually over 25), which almost always means two systems forward mail to each ot…
ssh: connect to host x port 22: Connection timed out
SSH packets get no reply — a firewall/security group drops them, the server is off, or the IP is wrong.
connect: No route to host (EHOSTUNREACH)
The destination host isn’t reachable — it’s down, on another subnet without routing, or a firewall rejects with “host unreachable…
Python: ConnectionRefusedError: [Errno 111] Connection refused — smtplib connect to localhost:25
Nothing is listening on the SMTP host and port your script connected to — often `localhost:25` because no host was given and no l…
SMTP: 554 5.7.1 Service unavailable; Client host [203.0.113.5] blocked / 554 5.7.1 <mail.example.com>: Client host rejected: Access denied
The server refused the connection from your IP because it is blocked by policy, not in the allowed list, or listed on a blacklist.
Python: smtplib.SMTPServerDisconnected: Connection unexpectedly closed
The server closed the connection before your script finished the SMTP conversation — a timeout, blocked IP, TLS mismatch or rate…
Cloudflare Error 523: Origin is unreachable
Cloudflare can’t find a network route to your origin server — the DNS record points to a wrong/unroutable IP or the origin networ…
Cloudflare HTTP 530 (with Error 1016 Origin DNS error / 1033 tunnel error)
Cloudflare returns 530 together with a 1xxx error; the 1xxx code (usually 1016 or 1033) tells you the real cause.
Cloudflare Error 1000: DNS points to prohibited IP
A proxied DNS record points at a Cloudflare IP address, creating a loop.
Cloudflare Error 1001: DNS resolution error
Cloudflare received the request but the hostname isn’t set up on Cloudflare (or the domain was just added and not active yet).
Cloudflare Error 1002: DNS points to Prohibited IP / Restricted
A record points to a private, reserved or Cloudflare IP that Cloudflare won’t proxy to.
Cloudflare Error 1003: Direct IP access not allowed
The request went to a Cloudflare IP address directly (or with a Host header for a domain not on Cloudflare).
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Popular: 500403400ORA-009331064Critical errorFileNotFoundException assemblyTypeError