SMTP 221 Closing Connection vs 450 Mailbox Unavailable
SMTP 221 (Closing Connection) is a 2xx Positive Completion response, while 450 (Mailbox Unavailable) is a 4xx Transient 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, 450 means that the requested mail action was not taken because the mailbox is temporarily unavailable. This could be due to the mailbox being locked by another process or a temporary policy restriction.
Descripción
The server is closing the transmission channel. This is the normal response to the QUIT command, indicating a graceful end to the SMTP session.
Cuándo lo verás
After sending the QUIT command at the end of a mail session. The server acknowledges the disconnect and the TCP connection will be closed.
Cómo solucionarlo
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.
Descripción
The requested mail action was not taken because the mailbox is temporarily unavailable. This could be due to the mailbox being locked by another process or a temporary policy restriction.
Cuándo lo verás
When the recipient's mailbox exists but is currently locked, busy, or subject to greylisting. The server is asking you to try again later.
Cómo solucionarlo
Retry delivery after a few minutes. If using greylisting, the second attempt will typically succeed. For persistent failures, check if the recipient's mailbox is full or disabled.
Diferencias clave
221 is a 2xx Positive Completion response, while 450 is a 4xx Transient 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 450: The requested mail action was not taken because the mailbox is temporarily unavailable. This could be due to the mailbox being locked by another process or a temporary policy restriction.
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 450 when when the recipient's mailbox exists but is currently locked, busy, or subject to greylisting. The server is asking you to try again later.
Cuándo usar cada uno
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 450 (Mailbox Unavailable): Retry delivery after a few minutes. If using greylisting, the second attempt will typically succeed. For persistent failures, check if the recipient's mailbox is full or disabled.