SMTP 421 Service Not Available vs 521 Host Does Not Accept Mail
SMTP 421 (Service Not Available) is a 4xx Transient Negative response, while 521 (Host Does Not Accept Mail) is a 5xx Permanent Negative response. 421 indicates 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. In contrast, 521 means that the host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
الوصف
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.
متى تراه
When connecting to a mail server that is overloaded, undergoing maintenance, or shutting down. The server cannot handle your request right now.
كيفية الإصلاح
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.
الوصف
The host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
متى تراه
When connecting to a server that has been intentionally configured to refuse all email, such as a domain that only sends but never receives mail.
كيفية الإصلاح
Verify you are connecting to the correct mail server by checking the domain's MX records. If the domain truly does not accept mail, contact the recipient through an alternative channel.
الفروق الرئيسية
421 is a 4xx Transient Negative response, while 521 is a 5xx Permanent Negative response.
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.
SMTP 521: The host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
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.
You encounter 521 when when connecting to a server that has been intentionally configured to refuse all email, such as a domain that only sends but never receives mail.
متى تستخدم أيًا منهما
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. For 521 (Host Does Not Accept Mail): Verify you are connecting to the correct mail server by checking the domain's MX records. If the domain truly does not accept mail, contact the recipient through an alternative channel.