# SMTP error codes explained: 421, 535, 550, 554 and more

> What each SMTP error means and how to fix it: 421, 450, 451, 530, 534, 535, 550 5.7.1, 553, 554, plus Nodemailer, PHPMailer and Python smtplib errors.

Source: https://formsmtp.com/guides/smtp-error-codes · Last updated November 2, 2026

## How to read an SMTP reply

Take `535 5.7.8 Username and Password not accepted`. It has three parts:

- **The reply code** (`535`), defined by RFC 5321. The first digit tells you what to do: `2` worked, `3` means continue, `4` is temporary (retrying can work), and `5` is permanent (fix something first).
- **The enhanced status code** (`5.7.8`), from RFC 3463. The middle number is the area: `1` addressing, `2` mailbox, `3` mail system, `4` network and routing, `5` protocol, `6` message content, `7` security or policy.
- **The text**, written by the provider. It’s usually the most useful part, so always log the whole line.

## SMTP replies

| Reply | Meaning | Likely cause when sending from an app | Fix |
| --- | --- | --- | --- |
| `421`, `421 4.7.0` | Service not available, closing the connection | The server is busy, throttling you, or distrusts your IP. Gmail uses 421 4.7.0 for low IP reputation, among others. | Retry later with a back-off. Send more slowly. |
| `450` | Mailbox unavailable, temporarily | Mailbox busy, or temporarily blocked by policy (including greylisting) | Retry. Most mail servers do this for you. |
| `451` | Aborted: local error in processing | A temporary problem on the receiving side | Retry later. |
| `452`, `452 4.5.3` | Insufficient system storage, or too many recipients | Too many recipients in one message | Send to fewer recipients per message. |
| `454 4.7.0` | Temporary authentication failure | Too many login attempts, or a server-side glitch | Wait, then retry. Don’t hammer the login. |
| `530 5.7.0 Must issue a STARTTLS command first` | The server requires TLS before anything else (RFC 3207) | Port 587 with STARTTLS disabled or ignored | Enable STARTTLS on 587, or use 465 with SSL on. |
| `530 5.7.0 Authentication required` | You must log in to send (RFC 4954) | No username or password configured | Add the SMTP username and password. Microsoft 365 says `5.7.57 Client not authenticated to send mail`. |
| `534 5.7.9` | Authentication mechanism too weak (RFC 4954) | Gmail: “Application-specific password required” | Use an App Password. See the [Gmail fix](https://formsmtp.com/guides/gmail-smtp-username-password-not-accepted). |
| `534 5.7.14` | Gmail: “Please log in through your web browser” | Google wants a browser sign-in before it accepts this login | Sign in to Gmail in a browser, then retry. |
| `535 5.7.8` | Authentication credentials invalid (RFC 4954) | Wrong username or password, or the provider wants an app password or SMTP key | See the [Gmail fix](https://formsmtp.com/guides/gmail-smtp-username-password-not-accepted) or the [settings for your provider](https://formsmtp.com/smtp-settings). |
| `535 5.7.139`, `535 5.7.3` | Microsoft 365 authentication unsuccessful | SMTP AUTH is disabled for the tenant or mailbox | See the [Microsoft 365 fix](https://formsmtp.com/guides/microsoft-365-smtp-auth-disabled). |
| `550 5.7.1` relaying denied | Rejected for policy; delivery not authorized | You didn’t log in, or the server doesn’t relay mail for you | Authenticate, and use your provider’s submission host on 587 or 465. |
| `550 5.7.26` | Gmail: unauthenticated email rejected due to the domain’s DMARC policy | The From domain fails SPF/DKIM alignment | See [emails going to spam](https://formsmtp.com/guides/contact-form-emails-going-to-spam). |
| `550 5.7.60` | Microsoft 365: no permission to send as this sender | From address differs from the signed-in mailbox | Send from the same mailbox, or grant Send As. |
| `550 5.4.5` | Gmail: daily user sending limit exceeded | Over Gmail’s daily limit | Wait up to 24 hours, or use a sending service. |
| `553 5.7.1` sender address rejected | Mailbox name not allowed | The From address isn’t one your login may use | Send from the login address or a verified alias. |
| `554 5.7.1` | Transaction failed; delivery not authorized | Refused by policy or a spam filter, or your IP is on a blocklist | Read the text that follows. Check SPF, DKIM and the content. |

## Library and connection errors

These come from your code or operating system, before the server says anything (or instead of it).

| Error | What happened | Fix |
| --- | --- | --- |
| `ETIMEDOUT`, `Connection timeout` | Nothing answered. A firewall dropped the connection. | The port is blocked. See [SMTP connection timed out](https://formsmtp.com/guides/smtp-port-blocked-connection-timeout). |
| `Greeting never received` (Nodemailer) | Connected, but no `220` greeting | Usually SSL off on port 465. Turn it on, or use 587. |
| `ECONNREFUSED` | The host answered “no” on that port | Wrong host or port, or nothing listening there. |
| `ENOTFOUND`, `getaddrinfo ENOTFOUND` | DNS couldn’t find the host name | A typo, or the wrong regional host (for example `smtp.zoho.eu` vs `smtp.zoho.com`). |
| `wrong version number` (OpenSSL) | TLS handshake on a port that speaks plain text first | SSL is on for port 587. Turn it off (STARTTLS), or use 465. |
| `self-signed certificate in certificate chain` | The certificate presented isn’t the provider’s | Something on the network intercepts TLS, or you’re connecting to the wrong host. Don’t disable verification to silence it. |
| `Invalid login: 535 …` (`EAUTH`) | Nodemailer’s wrapper around a `535` | Fix the credentials, as for 535. |
| `SMTP AUTH extension not supported by server.` (Python) | The server didn’t offer login. On 587, servers only offer it after STARTTLS. | Call `starttls()`, then `ehlo()`, then `login()`. Or use `SMTP_SSL` on 465. |
| `SMTP Error: Could not authenticate.` (PHPMailer) | The server rejected the login, usually with a 535 | Turn on debug output (below) to see the real reply. |
| `SMTP connect() failed.` (PHPMailer) | Couldn’t connect or start TLS | DNS, a firewall or a blocked port. Or the wrong mode: PHPMailer’s docs say SMTPS on 587 or STARTTLS on 465 “will not work”. |

## See the whole conversation

Most of these errors are easy once you see the server’s exact words. Turn on protocol logging:

```text
// Node.js (Nodemailer): print the whole SMTP conversation
nodemailer.createTransport({ host, port, secure, auth, logger: true, debug: true });

// PHP (PHPMailer)
$mail->SMTPDebug = SMTP::DEBUG_SERVER;

# Python (smtplib)
server.set_debuglevel(1)
```

Remove debug logging when you’re done. It can print your credentials (base64-encoded, which is not encryption) to the logs.

**Using FormSMTP?** FormSMTP logs the provider’s full reply for every failed send under *Logs*, adds a plain-English hint, and *Test connection* runs the connect, TLS and login steps before you save. It only needs your provider’s SMTP host, port and login. [Try it free](https://app.formsmtp.com/#/signup), or look up any provider on the [SMTP settings page](https://formsmtp.com/smtp-settings).

## Sources

Checked on the vendors’ own pages on November 2, 2026.

- [RFC 5321: Simple Mail Transfer Protocol](https://www.rfc-editor.org/rfc/rfc5321)
- [RFC 3463: Enhanced Mail System Status Codes](https://www.rfc-editor.org/rfc/rfc3463)
- [RFC 4954: SMTP Service Extension for Authentication](https://www.rfc-editor.org/rfc/rfc4954)
- [RFC 3207: SMTP Service Extension for Secure SMTP over TLS](https://www.rfc-editor.org/rfc/rfc3207)
- [RFC 8314: Use of TLS for Email Submission and Access](https://www.rfc-editor.org/rfc/rfc8314)
- [Google Workspace: Gmail SMTP errors and codes](https://knowledge.workspace.google.com/admin/support/troubleshooting/gmail-smtp-errors-and-codes)
- [Microsoft Learn: Fix issues with printers, scanners, and LOB apps that send email](https://learn.microsoft.com/en-us/troubleshoot/exchange/email-delivery/fix-issues-with-printers-scanners-and-lob-applications-that-send-email-using-off)
- [Microsoft Learn: Set up a device or application to send email using Microsoft 365](https://learn.microsoft.com/en-us/exchange/mail-flow-best-practices/how-to-set-up-a-multifunction-device-or-application-to-send-email-using-microsoft-365-or-office-365)
- [PHPMailer wiki: Troubleshooting](https://github.com/PHPMailer/PHPMailer/wiki/Troubleshooting)
- [Python docs: smtplib](https://docs.python.org/3/library/smtplib.html)

## FAQ

### What’s the difference between 4xx and 5xx SMTP errors?

A 4xx reply is temporary: the same message may go through if you retry later. A 5xx reply is permanent: retrying won’t help until you change something, such as the password, the From address or the recipient.

### What does 535 5.7.8 mean?

Authentication credentials invalid. The server rejected the username or password. With Gmail it usually means you need an App Password; with sending services it usually means you need an SMTP key instead of your account password.

### What does 550 5.7.1 relaying denied mean?

The server won’t pass your message on to another domain for you, usually because you didn’t log in or you’re connected to a server that isn’t your provider’s submission host.

### Why do I get “wrong version number” with Nodemailer?

secure: true is set on a STARTTLS port like 587. Use secure: true only with port 465, and secure: false with 587.

## More guides

- [Gmail 535-5.7.8 Username and Password not accepted: fix](https://formsmtp.com/guides/gmail-smtp-username-password-not-accepted)
- [535 5.7.139 SmtpClientAuthentication is disabled: fix](https://formsmtp.com/guides/microsoft-365-smtp-auth-disabled)
- [SMTP connection timed out (ETIMEDOUT): blocked port fix](https://formsmtp.com/guides/smtp-port-blocked-connection-timeout)
- [Contact form emails going to spam or not arriving: fixes](https://formsmtp.com/guides/contact-form-emails-going-to-spam)