SMTP 251 User Not Local vs 501 Syntax Error in Parameters
SMTP 251 (User Not Local) is a 2xx Positive Completion response, while 501 (Syntax Error in Parameters) 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, 501 means that the command was recognized but the parameters or arguments are syntactically invalid. The server understood what you wanted to do but could not parse the details.
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.
Quand vous le voyez
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.
Comment résoudre
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 command was recognized but the parameters or arguments are syntactically invalid. The server understood what you wanted to do but could not parse the details.
Quand vous le voyez
When a command like MAIL FROM or RCPT TO has a malformed email address, missing angle brackets, or invalid characters in the arguments.
Comment résoudre
Review the parameter format — email addresses should be enclosed in angle brackets (e.g., <[email protected]>). Check for stray spaces, missing colons, or encoding issues.
Différences clés
251 is a 2xx Positive Completion response, while 501 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 501: The command was recognized but the parameters or arguments are syntactically invalid. The server understood what you wanted to do but could not parse the details.
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 501 when when a command like MAIL FROM or RCPT TO has a malformed email address, missing angle brackets, or invalid characters in the arguments.
Quand utiliser lequel
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 501 (Syntax Error in Parameters): Review the parameter format — email addresses should be enclosed in angle brackets (e.g., <[email protected]>). Check for stray spaces, missing colons, or encoding issues.