SMTP 235 Authentication Successful vs 421 Service Not Available
SMTP 235 (Authentication Successful) is a 2xx Positive Completion response, while 421 (Service Not Available) is a 4xx Transient Negative response. 235 indicates that the client has been successfully authenticated using the AUTH command. The server will now accept mail commands from this authenticated session. 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.
คำอธิบาย
The client has been successfully authenticated using the AUTH command. The server will now accept mail commands from this authenticated session.
เมื่อคุณพบเห็น
After submitting valid credentials via the AUTH command (LOGIN, PLAIN, or other SASL mechanism). The server has verified your identity and you can now send mail.
วิธีแก้ไข
No fix needed — authentication succeeded. If you expected this but received a different code, double-check your username, password, and the authentication mechanism.
คำอธิบาย
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.
เมื่อคุณพบเห็น
When connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.
วิธีแก้ไข
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.
ความแตกต่างหลัก
235 is a 2xx Positive Completion response, while 421 is a 4xx Transient Negative response.
SMTP 235: The client has been successfully authenticated using the AUTH command. The server will now accept mail commands from this authenticated session.
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 235 when after submitting valid credentials via the AUTH command (LOGIN, PLAIN, or other SASL mechanism). The server has verified your identity and you can now send mail.
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.
ควรใช้อันไหนเมื่อไร
For 235 (Authentication Successful): No fix needed — authentication succeeded. If you expected this but received a different code, double-check your username, password, and the authentication mechanism. 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.