550 5.1.1: Permanent failure
550
The mailbox is unavailable or a policy rejected the request.
Next step: Read the enhanced code before deciding what to fix.
5.1.1
The destination mailbox doesn't exist.
Next step: Suppress this address until the recipient corrects it.
Common SMTP codes, explained
Each code has a permanent section link you can use in documentation, an issue, or an AI answer. These are selected common codes, not the complete SMTP registry.
| Code | Meaning | Next step |
|---|---|---|
| 250 | The command completed successfully. | Check which command received this reply. It doesn't prove inbox placement. |
| 354 | The server is ready for message content. | Continue the DATA exchange; this is not final acceptance. |
| 421 | The service is unavailable and is closing the connection. | Check service status and reconnect later. |
| 451 | A local processing problem interrupted the request. | Keep the job queued and investigate repeated failures. |
| 550 | The mailbox is unavailable or a policy rejected the request. | Read the enhanced code before deciding what to fix. |
| 552 | The request exceeded a storage allocation. | Check the provider's size and storage explanation. |
| 554 | The transaction failed, or SMTP service isn't available here. | Inspect the reply text and the command that failed. |
| 2.0.0 | Success, with no more specific status supplied. | Use delivery events for more context. |
| 4.2.2 | The recipient's mailbox is full. | Let the delivery queue handle a bounded retry. |
| 4.4.1 | The destination host didn't answer. | Check connectivity and later delivery attempts. |
| 4.7.0 | A temporary security or policy problem was reported. | Read the receiving provider's explanation before changing authentication. |
| 5.1.1 | The destination mailbox doesn't exist. | Suppress this address until the recipient corrects it. |
| 5.1.2 | The destination mail system is invalid or cannot accept mail. | Check the domain in the recipient address. |
| 5.3.4 | The message exceeds the system's size limit. | Reduce the message size before trying again. |
| 5.7.1 | The sender isn't permitted to deliver this message. | Investigate the stated policy; this code alone doesn't identify the cause. |
How to read an SMTP reply
The leading digit gives the result class: 2 means success, 3 needs more input, 4 indicates a temporary failure, and 5 indicates a permanent failure. A three-digit reply applies to a particular command. An enhanced code, such as 5.1.1, adds detail.
Does 550 always mean an address is invalid?
No. Read the enhanced code and provider text. A 550 with 5.1.1 points to the destination mailbox; a 550 with 5.7.1 points to authorization or filtering. Don’t remove a valid address based on 550 alone.
Can I retry a 4xx or 5xx error?
A temporary failure may clear later. A permanent failure calls for investigation or a correction before resubmitting. Check whether your provider already owns delivery retries so you don’t create a second message.
This lookup matches a small, reviewed reference and identifies the leading result class for unlisted codes. It doesn’t contact a mail server, validate a mailbox, or interpret provider-specific reply text. A code lookup cannot establish inbox placement.
Sources and references
Use these primary sources for the underlying behavior. Stampwing’s explanations and calculators are practical aids; your provider’s response and configuration still matter.