SMTP 251 User Not Local vs 504 Parameter Not Implemented
SMTP 251 (User Not Local) is a 2xx Positive Completion response, while 504 (Parameter Not Implemented) 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, 504 means that a command parameter is not implemented on this server. The command itself is valid but the specific parameter or extension you used is not supported.
描述
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.
何时出现
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.
如何修复
No fix needed — the message will be forwarded automatically. To avoid relaying, send directly to the address provided in the server's response.
描述
A command parameter is not implemented on this server. The command itself is valid but the specific parameter or extension you used is not supported.
何时出现
When using SMTP extensions or parameters that the server does not support, such as an unsupported AUTH mechanism or an unrecognized MAIL FROM parameter.
如何修复
Check the server's EHLO response for supported extensions and parameters. Use only the mechanisms and parameters listed in the server's capability advertisement.
主要区别
251 is a 2xx Positive Completion response, while 504 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 504: A command parameter is not implemented on this server. The command itself is valid but the specific parameter or extension you used is not supported.
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 504 when when using SMTP extensions or parameters that the server does not support, such as an unsupported AUTH mechanism or an unrecognized MAIL FROM parameter.
何时使用哪个
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 504 (Parameter Not Implemented): Check the server's EHLO response for supported extensions and parameters. Use only the mechanisms and parameters listed in the server's capability advertisement.