SMTP Response Codes
Complete reference of SMTP status codes returned by mail servers. These codes appear in your email logs and help diagnose delivery issues.
SMTP response codes follow a three-digit format. The first digit indicates the class: 2xx = success, 3xx = intermediate, 4xx = temporary failure, 5xx = permanent failure. Enhanced status codes (e.g., 5.1.1) provide more specific details.
2xxSuccess Codes
These codes indicate the mail server accepted the command or message.
| Code | Description | Details |
|---|---|---|
| 211 | System status | System status or help reply |
| 214 | Help message | Help message from the server |
| 220 | Service ready | The mail server is ready to accept connections |
| 221 | Closing connection | The server is closing the SMTP session |
| 235 | Authentication successful | SMTP authentication was accepted |
| 250 | OK | The requested action was completed successfully. Message accepted for delivery. |
| 251 | User not local | The recipient is not local but the server will forward the message |
| 252 | Cannot verify user | The server cannot verify the user but will accept and attempt delivery |
4xxTemporary Failure Codes
These codes indicate a temporary problem. The sending server should retry the message later.
| Code | Description | Details |
|---|---|---|
| 421 | Service not available | The server is temporarily unavailable or closing the connection. Often due to too many connections. |
| 450 | Mailbox unavailable | The recipient's mailbox is temporarily unavailable (busy or locked by another process). |
| 451 | Local error | The server encountered a local processing error. The message should be retried. |
| 452 | Insufficient storage | The server has run out of storage space or the recipient's mailbox is full. |
| 455 | Server unable to accommodate | The server cannot process the parameters at this time. Retry later. |
Enhanced 4xx Codes
| Code | Description |
|---|---|
| 421 4.3.0 | Mail server temporarily rejected message — too many messages or connections |
| 421 4.7.0 | Connection rate limited — too many connections from your IP |
| 450 4.1.8 | Sender rejected — bad reputation for your IP or domain |
| 450 4.2.1 | Recipient has disabled their mailbox |
| 452 4.2.2 | Recipient mailbox is over quota (full) |
| 451 4.7.1 | Greylisting — try again in a few minutes |
5xxPermanent Failure Codes
These codes indicate a permanent error. The message should not be retried to the same recipient without changes.
| Code | Description | Details |
|---|---|---|
| 500 | Syntax error | The command was not recognized or has a syntax error |
| 501 | Parameter error | Syntax error in the command parameters or arguments |
| 502 | Command not implemented | The server does not support this command |
| 503 | Bad sequence | Commands were sent in the wrong order |
| 504 | Parameter not implemented | A command parameter is not implemented on this server |
| 550 | Mailbox unavailable | The recipient's mailbox does not exist or email was rejected by policy |
| 551 | User not local | The recipient is not local to this server and forwarding is not allowed |
| 552 | Storage exceeded | The message exceeds the allocaed storage for the mailbox |
| 553 | Mailbox name invalid | The recipient address format is invalid |
| 554 | Transaction failed | General rejection — can be content, authentication, or policy related |
Enhanced 5xx Codes
| Code | Description |
|---|---|
| 550 5.1.1 | Recipient address does not exist |
| 550 5.1.2 | Recipient domain does not exist |
| 550 5.4.1 | Recipient address rejected — no relay allowed |
| 550 5.7.1 | Message rejected due to policy (SPF/DKIM/DMARC failure or blocked) |
| 550 5.7.25 | PTR record for sending IP not found |
| 550 5.7.26 | Email failed SPF or DMARC authentication |
| 553 5.1.3 | Invalid email address format |
| 554 5.7.5 | Cryptographic failure (DKIM verification error) |
| 554 5.7.9 | DKIM signature verification failed |
| 556 5.1.10 | Recipient address has null MX — domain does not accept email |