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.
Описание
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.
Когда вы это видите
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.
Как исправить
No fix needed — the message will be forwarded automatically. To avoid relaying, send directly to the address provided in the server's response.
Описание
The authentication credentials provided are invalid. The username, password, or authentication mechanism was rejected by the server.
Когда вы это видите
After submitting incorrect credentials via the AUTH command. The username or password does not match any account on the server.
Как исправить
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 If using an app password (e.g., Gmail, Outlook), ensure it is current and has not been revoked.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.).
Ключевые различия
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.
Когда что использовать
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.).