Error map

Helo command rejected: need fully-qualified hostname SMTP: 504 5.5.2 <localhost>: Helo command rejected: need fully-qualified hostname

Email / SMTP · layer 2, Network

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Centre: this error. First ring: errors it is linked with (7). 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.

2 · Network 4 · Application & API 6 · Platform & cloud
Helo command rejected: need fully-qualified hostname — SMTP: 504 5.5.2 <localhost>: Helo command rejected: need fully-qualified hostname (Email / SMTP)Helo comman…550 5.7.25 PTR record required — 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 (Email / SMTP)550 5.7.25 PTR reco…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)Postfix: Host or do…421 4.7.0 Try again later (rate limited) — 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 (Email / SMTP)421 4.7.0 Try again…450 4.7.1 Greylisted, please try again later — SMTP: 450 4.7.1 <user@example.com>: Recipient address rejected: Greylisted, see http://postgrey.schweikert.ch/help/example.com.html (Email / SMTP)450 4.7.1 Greyliste…554 5.7.1 Service unavailable; client host rejected — 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 (Email / SMTP)554 5.7.1 Service u…SMTPServerDisconnected: Connection unexpectedly closed — Python: smtplib.SMTPServerDisconnected: Connection unexpectedly closed (Email / SMTP)SMTPServerDisconnec…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)No MX record / Doma…cURL error 6 — cURL error 6: Could not resolve host: api.example.com (PHP)cURL error 6sudo: unable to resolve host — sudo: unable to resolve host <hostname>: Name or service not known (Linux)sudo: unable to res…Temporary failure in name resolution — ping: google.com: Temporary failure in name resolution (Linux)Temporary failure i…DKIM: no key for signature — DKIM: dkim=permerror (no key for signature) / key not found in DNS selector._domainkey (Email / SMTP)DKIM: no key for si…DMARC: no record found — DMARC: dmarc=none (no DMARC record found for example.com) — bulk sender requirements not met (Email / SMTP)DMARC: no record fo…Postfix: status=deferred (connect ... Connection timed out) — Postfix: status=deferred (connect to gmail-smtp-in.l.google.com[142.250.27.26]:25: Connection timed out) (Email / SMTP)Postfix: status=def…SMTP Error: Could not authenticate — WordPress / PHPMailer: SMTP Error: Could not authenticate / SMTP connect() failed (WordPress)SMTP Error: Could n…550 5.7.1 — 550 5.7.1 Relaying denied / Message rejected as spam / Unauthenticated email not accepted (Email / SMTP)550 5.7.1550 5.7.26 — 550 5.7.26 This mail has been blocked because the sender is unauthenticated (SPF/DKIM/DMARC) (Email / SMTP)550 5.7.26451 4.3.0 Local error in processing — SMTP: 451 4.3.0 Error: queue file write error / 451 Requested action aborted: local error in processing (Email / SMTP)451 4.3.0 Local err…452 4.2.2 Mailbox full / over quota — SMTP: 452 4.2.2 The email account that you tried to reach is over quota (Email / SMTP)452 4.2.2 Mailbox f…452 4.5.3 Too many recipients — SMTP: 452 4.5.3 Your message has too many recipients / 452 Too many recipients received this hour (Email / SMTP)452 4.5.3 Too many …552 5.3.4 Message size exceeds fixed limit — SMTP: 552 5.3.4 Message size exceeds fixed maximum message size (Email / SMTP)552 5.3.4 Message s…550 5.7.1 Blocked using zen.spamhaus.org — SMTP: 550 5.7.1 Service unavailable; Client host [203.0.113.5] blocked using zen.spamhaus.org (Email / SMTP)550 5.7.1 Blocked u…550 5.4.1 Recipient address rejected: Access denied — SMTP: 550 5.4.1 Recipient address rejected: Access denied. AS(201806281) [DB5EUR03FT015.eop-EUR03.prod.protection.outlook.com] (Email / SMTP)550 5.4.1 Recipient…SSL: WRONG_VERSION_NUMBER (SMTP port) — SMTP: [SSL: WRONG_VERSION_NUMBER] wrong version number — SMTP over SSL on a STARTTLS port (Email / SMTP)SSL: WRONG_VERSION_…SMTPAuthenticationError (Python) — Python: smtplib.SMTPAuthenticationError: (535, b'5.7.8 Username and Password not accepted') (Email / SMTP)SMTPAuthenticationE…ConnectionRefusedError [Errno 111] (SMTP) — Python: ConnectionRefusedError: [Errno 111] Connection refused — smtplib connect to localhost:25 (Email / SMTP)ConnectionRefusedEr…550 5.1.1 — 550 5.1.1 The email account that you tried to reach does not exist (User unknown) (Email / SMTP)550 5.1.1554 5.4.14 Hop count exceeded - possible mail loop — SMTP: 554 5.4.14 Hop count exceeded - possible mail loop (Email / SMTP)554 5.4.14 Hop coun…

Directly related (7)

550 5.7.25 PTR record requiredEmail / SMTP

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: Host or domain name not foundEmail / SMTP

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.

421 4.7.0 Try again later (rate limited)Email / SMTP

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…

450 4.7.1 Greylisted, please try again laterEmail / SMTP

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…

554 5.7.1 Service unavailable; client host rejectedEmail / SMTP

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.

SMTPServerDisconnected: Connection unexpectedly closedEmail / SMTP

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…

No MX record / Domain not foundEmail / SMTP

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.

Two steps away (20)

cURL error 6PHP

cURL error 6: Could not resolve host: api.example.com

PHP cURL couldn’t resolve the hostname via DNS.

sudo: unable to resolve hostLinux

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.

Temporary failure in name resolutionLinux

ping: google.com: Temporary failure in name resolution

The Linux host can’t resolve domain names — no working DNS server is configured or reachable.

DKIM: no key for signatureEmail / SMTP

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…

DMARC: no record foundEmail / SMTP

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…

Postfix: status=deferred (connect ... Connection timed out)Email / SMTP

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.

SMTP Error: Could not authenticateWordPress

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 5.7.1Email / SMTP

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 5.7.26Email / SMTP

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).

451 4.3.0 Local error in processingEmail / SMTP

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.

452 4.2.2 Mailbox full / over quotaEmail / SMTP

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.

452 4.5.3 Too many recipientsEmail / SMTP

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.

552 5.3.4 Message size exceeds fixed limitEmail / SMTP

SMTP: 552 5.3.4 Message size exceeds fixed maximum message size

The message, including attachments after base64 encoding, is larger than the receiving or sending server accepts.

550 5.7.1 Blocked using zen.spamhaus.orgEmail / SMTP

SMTP: 550 5.7.1 Service unavailable; Client host [203.0.113.5] blocked using zen.spamhaus.org

The receiving server checked your IP in a DNS blocklist (Spamhaus, Barracuda, SORBS…) and refused your mail because it is listed.

550 5.4.1 Recipient address rejected: Access deniedEmail / SMTP

SMTP: 550 5.4.1 Recipient address rejected: Access denied. AS(201806281) [DB5EUR03FT015.eop-EUR03.prod.protection.outlook.com]

Microsoft 365 / Exchange Online rejected the message because the recipient address does not exist in that tenant (Directory Based…

SSL: WRONG_VERSION_NUMBER (SMTP port)Email / SMTP

SMTP: [SSL: WRONG_VERSION_NUMBER] wrong version number — SMTP over SSL on a STARTTLS port

Your client tried to start TLS immediately on a port where the server speaks plain SMTP first (or the reverse), so the handshake…

SMTPAuthenticationError (Python)Email / SMTP

Python: smtplib.SMTPAuthenticationError: (535, b'5.7.8 Username and Password not accepted')

`smtp.login()` was rejected by the server — wrong credentials, wrong password type, or authentication not allowed for that accoun…

ConnectionRefusedError [Errno 111] (SMTP)Email / SMTP

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…

550 5.1.1Email / SMTP

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.

554 5.4.14 Hop count exceeded - possible mail loopEmail / SMTP

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…

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