Error map
SMTPServerDisconnected: Connection unexpectedly closed Python: smtplib.SMTPServerDisconnected: Connection unexpectedly closed
Email / SMTP · layer 2, Network
Centre: this error. First ring: errors it is linked with (8). 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 (8)
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: 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: [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…
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.
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: 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.
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…
Two steps away (24)
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: 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.
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.
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 5.1.1 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: 553 5.7.1 <noreply@example.com>: Sender address rejected: not owned by user user@example.com
The SMTP server only lets an authenticated user send as addresses they own, and the From/envelope sender does not match the login.
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.
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.
Nodemailer: Error: Greeting never received (code: 'ETIMEDOUT', command: 'CONN')
The TCP connection was made but the server never sent its SMTP `220` banner — usually a TLS mode mismatch or a firewall/proxy swa…
Nodemailer: Error: self signed certificate in certificate chain (code: 'ESOCKET') / unable to verify the first certificate
Nodemailer refused the SMTP server's TLS certificate because it is self-signed, expired, hostname-mismatched or missing its inter…
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…
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…
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…
Nodemailer: Error: connect ETIMEDOUT 142.250.27.108:465 (code: 'ETIMEDOUT', command: 'CONN')
Nodemailer could not open a connection to the SMTP server before the timeout — a blocked port, wrong host, or network issue.
PHP: sh: 1: /usr/sbin/sendmail: not found — mail() returns false
PHP's `mail()` calls the `sendmail_path` program, but it is not installed or the path in php.ini is wrong.
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: 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…
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.
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: 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.
SMTP: 535 5.7.139 Authentication unsuccessful, basic authentication is disabled / SmtpClientAuthentication is disabled for the Tenant
Microsoft 365 turned off password-based SMTP AUTH for your mailbox or tenant; your client must use OAuth or the setting must be r…
Nodemailer: Error: Invalid login: 535-5.7.8 Username and Password not accepted (code: 'EAUTH', responseCode: 535)
The SMTP server rejected the credentials passed to Nodemailer.
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