SMTP 251 User Not Local vs 530 Authentication Required
SMTP 251 (User Not Local) is a 2xx Positive Completion response, while 530 (Authentication Required) is a 5xx Permanent Negative response. 251 indicates that the recipient is not local to the server, but it will forward the message to the specified forwarding address. The server acts as a relay for this recipient. In contrast, 530 means that the server requires authentication before accepting mail commands. You must issue the AUTH command with valid credentials before attempting to send mail.
説明
The recipient is not local to the server, but it will forward the message to the specified forwarding address. The server acts as a relay for this recipient.
このコードが表示される場合
When sending to a recipient whose mailbox is hosted on a different server. The current server knows the forwarding address and will relay the message on your behalf.
解決方法
No fix needed — the message will be forwarded automatically. To avoid relaying, send directly to the address provided in the server's response.
説明
The server requires authentication before accepting mail commands. You must issue the AUTH command with valid credentials before attempting to send mail.
このコードが表示される場合
When trying to send mail through a server that requires authentication (typical for submission on port 587) without first logging in with the AUTH command.
解決方法
Authenticate using the AUTH command with your username and password before sending MAIL FROM. Ensure your email client is configured with the correct SMTP credentials and authentication method.
主な違い
251 is a 2xx Positive Completion response, while 530 is a 5xx Permanent Negative response.
SMTP 251: The recipient is not local to the server, but it will forward the message to the specified forwarding address. The server acts as a relay for this recipient.
SMTP 530: The server requires authentication before accepting mail commands. You must issue the AUTH command with valid credentials before attempting to send mail.
You encounter 251 when when sending to a recipient whose mailbox is hosted on a different server. The current server knows the forwarding address and will relay the message on your behalf.
You encounter 530 when when trying to send mail through a server that requires authentication (typical for submission on port 587) without first logging in with the AUTH command.
どちらをいつ使うか
For 251 (User Not Local): No fix needed — the message will be forwarded automatically. To avoid relaying, send directly to the address provided in the server's response. For 530 (Authentication Required): Authenticate using the AUTH command with your username and password before sending MAIL FROM. Ensure your email client is configured with the correct SMTP credentials and authentication method.