SMTP 251 User Not Local vs 451 Local Error
SMTP 251 (User Not Local) is a 2xx Positive Completion response, while 451 (Local Error) is a 4xx Transient Negative response. 251 indicates that the recipient is not local to the server, but it will forward the message to the specified forwarding address. The server acts as a relay for this recipient. In contrast, 451 means that the requested action was aborted due to a local error in processing. The server encountered an internal issue that prevented it from completing the command.
Description
The recipient is not local to the server, but it will forward the message to the specified forwarding address. The server acts as a relay for this recipient.
When You See It
When sending to a recipient whose mailbox is hosted on a different server. The current server knows the forwarding address and will relay the message on your behalf.
How to Fix
No fix needed — the message will be forwarded automatically. To avoid relaying, send directly to the address provided in the server's response.
Description
The requested action was aborted due to a local error in processing. The server encountered an internal issue that prevented it from completing the command.
When You See It
When the receiving server hits an internal error such as a database failure, DNS lookup timeout, or anti-spam filter processing error.
How to Fix
Retry the message later. If you control the server, check its logs for the specific error — common causes include DNS resolution failures, disk I/O errors, or database connectivity issues.
Key Differences
251 is a 2xx Positive Completion response, while 451 is a 4xx Transient Negative response.
SMTP 251: The recipient is not local to the server, but it will forward the message to the specified forwarding address. The server acts as a relay for this recipient.
SMTP 451: The requested action was aborted due to a local error in processing. The server encountered an internal issue that prevented it from completing the command.
You encounter 251 when when sending to a recipient whose mailbox is hosted on a different server. The current server knows the forwarding address and will relay the message on your behalf.
You encounter 451 when when the receiving server hits an internal error such as a database failure, DNS lookup timeout, or anti-spam filter processing error.
When to Use Which
For 251 (User Not Local): No fix needed — the message will be forwarded automatically. To avoid relaying, send directly to the address provided in the server's response. For 451 (Local Error): Retry the message later. If you control the server, check its logs for the specific error — common causes include DNS resolution failures, disk I/O errors, or database connectivity issues.