SMTP 221 Closing Connection vs 521 Host Does Not Accept Mail
SMTP 221 (Closing Connection) is a 2xx Positive Completion response, while 521 (Host Does Not Accept Mail) is a 5xx Permanent Negative response. 221 indicates that the server is closing the transmission channel. This is the normal response to the QUIT command, indicating a graceful end to the SMTP session. 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.
설명
The server is closing the transmission channel. This is the normal response to the QUIT command, indicating a graceful end to the SMTP session.
이 코드를 보게 되는 경우
After sending the QUIT command at the end of a mail session. The server acknowledges the disconnect and the TCP connection will be closed.
해결 방법
No fix needed — this is the expected response when ending an SMTP session. If you see this unexpectedly, the server may be shutting down or timing out idle connections.
설명
The host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
이 코드를 보게 되는 경우
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.
해결 방법
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.
주요 차이점
221 is a 2xx Positive Completion response, while 521 is a 5xx Permanent Negative response.
SMTP 221: The server is closing the transmission channel. This is the normal response to the QUIT command, indicating a graceful end to the SMTP session.
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 221 when after sending the QUIT command at the end of a mail session. The server acknowledges the disconnect and the TCP connection will be closed.
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.
언제 어떤 것을 사용할지
For 221 (Closing Connection): No fix needed — this is the expected response when ending an SMTP session. If you see this unexpectedly, the server may be shutting down or timing out idle connections. 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.