SMTP 221 Closing Connection vs 551 User Not Local
SMTP 221 (Closing Connection) is a 2xx Positive Completion response, while 551 (User Not Local) 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, 551 means that the recipient is not local to this server and the server does not accept mail for forwarding. Unlike 251, this is a rejection — the server will not relay the message.
คำอธิบาย
The server is closing the transmission channel. This is the normal response to the QUIT command, indicating a graceful end to the SMTP session.
เมื่อคุณพบเห็น
After sending the QUIT command at the end of a mail session. The server acknowledges the disconnect and the TCP connection will be closed.
วิธีแก้ไข
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.
คำอธิบาย
The recipient is not local to this server and the server does not accept mail for forwarding. Unlike 251, this is a rejection — the server will not relay the message.
เมื่อคุณพบเห็น
When sending to a recipient that has moved or whose mailbox is hosted elsewhere, and this server refuses to forward the message to the new location.
วิธีแก้ไข
Check the server's response for a forwarding address and resend directly to that address. Update your contact records if the recipient has permanently moved to a new mail server.
ความแตกต่างหลัก
221 is a 2xx Positive Completion response, while 551 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 551: The recipient is not local to this server and the server does not accept mail for forwarding. Unlike 251, this is a rejection — the server will not relay the message.
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 551 when when sending to a recipient that has moved or whose mailbox is hosted elsewhere, and this server refuses to forward the message to the new location.
ควรใช้อันไหนเมื่อไร
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 551 (User Not Local): Check the server's response for a forwarding address and resend directly to that address. Update your contact records if the recipient has permanently moved to a new mail server.