SMTP 211 System Status vs 421 Service Not Available
SMTP 211 (System Status) is a 2xx Positive Completion response, while 421 (Service Not Available) is a 4xx Transient Negative response. 211 indicates that a system status reply providing information about the mail server's current state. This is typically sent in response to a HELP or system inquiry command. 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.
描述
A system status reply providing information about the mail server's current state. This is typically sent in response to a HELP or system inquiry command.
何时出现
When querying the mail server for its status or capabilities. The server is sharing diagnostic or informational details about itself.
如何修复
No fix needed — this is an informational response. If the status indicates a problem, review the server's configuration or contact the mail administrator.
描述
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.
何时出现
When connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.
如何修复
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.
主要区别
211 is a 2xx Positive Completion response, while 421 is a 4xx Transient Negative response.
SMTP 211: A system status reply providing information about the mail server's current state. This is typically sent in response to a HELP or system inquiry command.
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 211 when when querying the mail server for its status or capabilities. The server is sharing diagnostic or informational details about itself.
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.
何时使用哪个
For 211 (System Status): No fix needed — this is an informational response. If the status indicates a problem, review the server's configuration or contact the mail administrator. 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.