SMTP 251 User Not Local vs 553 Mailbox Name Invalid
SMTP 251 (User Not Local) is a 2xx Positive Completion response, while 553 (Mailbox Name Invalid) is a 5xx Permanent 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, 553 means that the requested action was not taken because the mailbox name is syntactically incorrect. The email address does not conform to the expected format.
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 not taken because the mailbox name is syntactically incorrect. The email address does not conform to the expected format.
When You See It
When the sender or recipient address contains invalid characters, improper formatting, or violates the server's naming conventions.
How to Fix
Check the email address format — ensure it follows the standard [email protected] pattern with valid characters. Remove any spaces, special characters, or encoding issues from the address.
Key Differences
251 is a 2xx Positive Completion response, while 553 is a 5xx Permanent 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 553: The requested action was not taken because the mailbox name is syntactically incorrect. The email address does not conform to the expected format.
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 553 when when the sender or recipient address contains invalid characters, improper formatting, or violates the server's naming conventions.
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 553 (Mailbox Name Invalid): Check the email address format — ensure it follows the standard [email protected] pattern with valid characters. Remove any spaces, special characters, or encoding issues from the address.