SMTP 252 Cannot Verify User vs 504 Parameter Not Implemented
SMTP 252 (Cannot Verify User) is a 2xx Positive Completion response, while 504 (Parameter Not Implemented) is a 5xx Permanent Negative response. 252 indicates that the server cannot verify the user but will accept the message and attempt delivery. This is often returned in response to VRFY when the server intentionally hides user information. 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 server cannot verify the user but will accept the message and attempt delivery. This is often returned in response to VRFY when the server intentionally hides user information.
何时出现
When using the VRFY command to check if a recipient exists. The server refuses to confirm or deny the address, typically as a spam-prevention measure.
如何修复
No fix needed — the server will still attempt delivery. If you need to verify addresses, contact the mail administrator or rely on bounce-back messages instead.
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
252 is a 2xx Positive Completion response, while 504 is a 5xx Permanent Negative response.
SMTP 252: The server cannot verify the user but will accept the message and attempt delivery. This is often returned in response to VRFY when the server intentionally hides user information.
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 252 when when using the VRFY command to check if a recipient exists. The server refuses to confirm or deny the address, typically as a spam-prevention measure.
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 252 (Cannot Verify User): No fix needed — the server will still attempt delivery. If you need to verify addresses, contact the mail administrator or rely on bounce-back messages instead. 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.