Error map
No MX record / Domain not found SMTP: 550 5.1.2 Domain not found / 554 5.4.4 Unable to resolve MX / Host or domain name not found
Email / SMTP · layer 2, Network
Centre: this error. First ring: errors it is linked with (9). 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 (9)
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: 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 (Host or domain name not found. Name service error for name=example.com type=A: Host not found, try again)
Postfix could not resolve the destination domain through DNS — the resolver is down or unreachable, or the domain does not exist.
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: 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…
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…
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.
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…
Two steps away (24)
PHP mail() returns false / emails not delivered
PHP’s mail() function failed or emails never arrive (spam or rejected).
SES MessageRejected: Email address is not verified. The following identities failed the check in region
SES refused to send because the From address/domain isn’t verified — or the account is still in the SES sandbox, where recipients…
Contact Form 7: There was an error trying to send your message. Please try again later.
The form submitted, but wp_mail() failed to send — the server can’t send mail or the mail settings are wrong.
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.
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: 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.
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…
WordPress / PHPMailer: SMTP Error: Could not authenticate / SMTP connect() failed
The SMTP server rejected the login or couldn’t be reached — wrong credentials, app passwords required, wrong port/encryption, or…
550 5.7.1 Relaying denied / Message rejected as spam / Unauthenticated email not accepted
The receiving (or your outgoing) mail server refused the message — relaying isn’t allowed, the sender isn’t authenticated, or it…
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).
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: 452 4.2.2 The email account that you tried to reach is over quota
The recipient's mailbox has exceeded its storage quota, so the server will not accept new mail for now.
SMTP: 452 4.5.3 Your message has too many recipients / 452 Too many recipients received this hour
The server limits how many recipients a single message or sender may have; yours exceeded it.
TypeError: Missing parameter name at 1 (path-to-regexp)
An Express 5 (path-to-regexp v8) route uses old wildcard syntax like '*' or '/:id?'.
connect: Network is unreachable (ENETUNREACH)
The host has no route to the destination network — no default gateway, interface down, or trying IPv6 without IPv6 connectivity.
WordPress REST API: {"code":"rest_no_route","message":"No route was found matching the URL and request method."}
The REST endpoint doesn’t exist for that URL and HTTP method — wrong namespace/route, plugin not active, or route registered too…
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.
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…
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…
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