SMTP 251 User Not Local vs 535 Authentication Failed
SMTP 251 (User Not Local) is a 2xx Positive Completion response, while 535 (Authentication Failed) 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, 535 means that the authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server.
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 authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server.
このコードが表示される場合
After submitting incorrect credentials via the AUTH command. The username or password does not match any account on the server.
解決方法
Double-check your username and password. If using an app password (e.g., Gmail, Outlook), ensure it is current and has not been revoked. Also verify the correct SASL mechanism (PLAIN, LOGIN, etc.).
主な違い
251 is a 2xx Positive Completion response, while 535 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 535: The authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server.
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 535 when after submitting incorrect credentials via the AUTH command. The username or password does not match any account on the server.
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 535 (Authentication Failed): Double-check your username and password. If using an app password (e.g., Gmail, Outlook), ensure it is current and has not been revoked. Also verify the correct SASL mechanism (PLAIN, LOGIN, etc.).