SMTP 252 Cannot Verify User vs 421 Service Not Available
SMTP 252 (Cannot Verify User) is a 2xx Positive Completion response, while 421 (Service Not Available) is a 4xx Transient 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, 421 means that the SMTP service is not available and the server is closing the transmission channel. This may be sent as a greeting or during a session if the server needs to shut down.
Mô tả
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.
Khi bạn thấy mã này
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.
Cách khắc phục
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.
Mô tả
The SMTP service is not available and the server is closing the transmission channel. This may be sent as a greeting or during a session if the server needs to shut down.
Khi bạn thấy mã này
When connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.
Cách khắc phục
Retry the connection after a delay. If the issue persists, check server status and resources (CPU, memory, disk). Rate limiting or greylisting may also trigger this response.
Sự khác biệt chính
252 is a 2xx Positive Completion response, while 421 is a 4xx Transient 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 421: The SMTP service is not available and the server is closing the transmission channel. This may be sent as a greeting or during a session if the server needs to shut down.
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 421 when when connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.
Khi nào dùng cái nào
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 421 (Service Not Available): Retry the connection after a delay. If the issue persists, check server status and resources (CPU, memory, disk). Rate limiting or greylisting may also trigger this response.