SMTP 220 Service Ready vs 421 Service Not Available
SMTP 220 (Service Ready) is a 2xx Positive Completion response, while 421 (Service Not Available) is a 4xx Transient Negative response. 220 indicates that the SMTP server is ready to begin the mail transaction. This greeting is the first response a client receives upon connecting, and it typically includes the server's hostname and software identificati 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.
Descrição
The SMTP server is ready to begin the mail transaction. This greeting is the first response a client receives upon connecting, and it typically includes the server's hostname and software identification.
Quando você o vê
Immediately after establishing a TCP connection to the mail server on port 25, 465, or 587. This is the server's welcome banner confirming it is ready to accept commands.
Como corrigir
No fix needed — this confirms a successful connection. If you do not receive this greeting, check that the server is running, the port is correct, and no firewall is blocking the connection.
Descrição
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.
Quando você o vê
When connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.
Como corrigir
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.
Diferenças principais
220 is a 2xx Positive Completion response, while 421 is a 4xx Transient Negative response.
SMTP 220: The SMTP server is ready to begin the mail transaction. This greeting is the first response a client receives upon connecting, and it typically includes the server's hostname and software identificati
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 220 when immediately after establishing a TCP connection to the mail server on port 25, 465, or 587. This is the server's welcome banner confirming it is ready to accept commands.
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.
Quando usar qual
For 220 (Service Ready): No fix needed — this confirms a successful connection. If you do not receive this greeting, check that the server is running, the port is correct, and no firewall is blocking the connection. 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.