SMTP Error Codes¶
When emails fail to deliver, you'll often see an SMTP error code. This guide explains what they mean and how to fix them.
How SMTP Codes Work¶
SMTP codes are 3-digit numbers:
- 2xx - Success
- 4xx - Temporary failure (try again later)
- 5xx - Permanent failure (won't succeed without changes)
2xx - Success Codes¶
| Code | Meaning |
|---|---|
| 220 | Server ready |
| 221 | Server closing connection |
| 250 | Requested action completed |
| 251 | User not local; message forwarded |
| 252 | Cannot verify user, but will try delivery |
4xx - Temporary Failures¶
These errors mean "try again later" - the server might accept the message on a retry.
| Code | Meaning | What to Do |
|---|---|---|
| 421 | Service not available | Server is temporarily down; retry later |
| 450 | Mailbox unavailable | Mailbox busy or temporarily blocked; retry |
| 451 | Local error in processing | Server had a temporary problem; retry |
| 452 | Insufficient storage | Recipient's mailbox is full; wait and retry |
5xx - Permanent Failures¶
These errors mean the message will never be delivered without changes.
500-509: Syntax Errors¶
| Code | Meaning |
|---|---|
| 500 | Syntax error, command not recognised |
| 501 | Syntax error in parameters |
| 502 | Command not implemented |
| 503 | Bad sequence of commands |
| 504 | Command parameter not implemented |
510-519: Address Errors¶
| Code | Meaning | Common Cause |
|---|---|---|
| 550 | Mailbox not found | Email address doesn't exist |
| 551 | User not local | Recipient moved; try different address |
| 552 | Storage exceeded | Recipient's mailbox is full |
| 553 | Mailbox name invalid | Typo in email address |
| 554 | Transaction failed | Generic rejection; often spam-related |
Enhanced Status Codes¶
Modern servers use enhanced codes like 5.1.1 for more detail:
X.1.X - Address Issues¶
| Code | Meaning |
|---|---|
| X.1.0 | Other address status |
| X.1.1 | Bad destination mailbox address |
| X.1.2 | Bad destination system address |
| X.1.3 | Bad destination mailbox address syntax |
| X.1.4 | Destination mailbox address ambiguous |
| X.1.5 | Destination mailbox address valid |
| X.1.6 | Mailbox has moved |
| X.1.7 | Bad sender's mailbox address syntax |
| X.1.8 | Bad sender's system address |
X.2.X - Mailbox Issues¶
| Code | Meaning |
|---|---|
| X.2.1 | Mailbox disabled |
| X.2.2 | Mailbox full |
| X.2.3 | Message length exceeds limit |
| X.2.4 | Mailing list expansion problem |
X.4.X - Network Issues¶
| Code | Meaning |
|---|---|
| X.4.0 | Other network or routing status |
| X.4.1 | No answer from host |
| X.4.2 | Bad connection |
| X.4.3 | Routing server failure |
| X.4.4 | Unable to route |
| X.4.5 | Network congestion |
| X.4.6 | Routing loop detected |
| X.4.7 | Delivery time expired |
X.5.X - Protocol Issues¶
| Code | Meaning |
|---|---|
| X.5.0 | Other protocol status |
| X.5.1 | Invalid command |
| X.5.2 | Syntax error |
| X.5.3 | Too many recipients |
| X.5.4 | Invalid command arguments |
X.7.X - Security/Policy Issues¶
| Code | Meaning |
|---|---|
| X.7.0 | Other security status |
| X.7.1 | Delivery not authorised |
| X.7.2 | Mailing list expansion prohibited |
| X.7.3 | Security conversion required |
| X.7.4 | Security features not supported |
| X.7.5 | Cryptographic failure |
| X.7.6 | Cryptographic algorithm not supported |
| X.7.7 | Message integrity failure |
| X.7.23 | SPF validation failed |
| X.7.24 | SPF validation error |
| X.7.25 | Reverse DNS validation failed |
| X.7.26 | Multiple authentication failures |
Common Bounce Messages Explained¶
"550 5.1.1 User unknown"¶
The email address doesn't exist. Check for typos.
"550 5.7.1 Relaying denied"¶
You're trying to send through a server that won't relay for you. Check your SMTP authentication settings.
"552 5.2.2 Mailbox full"¶
The recipient's mailbox has no space. They need to delete emails or increase their quota.
"554 5.7.1 Message rejected due to spam content"¶
Your email was flagged as spam. Check your content, links, and attachments.
"421 Try again later"¶
The receiving server is temporarily unavailable. Your mail server will automatically retry.
"550 SPF check failed"¶
Your sending server isn't authorised in the domain's SPF record. Contact the domain administrator.
Troubleshooting Tips¶
- Check the email address - Typos are the most common cause of bounces
- Review bounce details - The full error message usually explains the problem
- Check your SPF/DKIM - Authentication failures cause many rejections
- Look at send reputation - If you're blocked, you may be on a blacklist
- Retry later for 4xx errors - These are temporary and often resolve themselves