SMTP 235 Authentication Successful vs 251 User Not Local
Both SMTP 235 (Authentication Successful) and 251 (User Not Local) belong to the 2xx Positive Completion category. 235 indicates that the client has been successfully authenticated using the AUTH command. The server will now accept mail commands from this authenticated session. Meanwhile, 251 means 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.
Descripción
The client has been successfully authenticated using the AUTH command. The server will now accept mail commands from this authenticated session.
Cuándo lo verás
After submitting valid credentials via the AUTH command (LOGIN, PLAIN, or other SASL mechanism). The server has verified your identity and you can now send mail.
Cómo solucionarlo
No fix needed — authentication succeeded. If you expected this but received a different code, double-check your username, password, and the authentication mechanism.
Descripción
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.
Cuándo lo verás
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.
Cómo solucionarlo
No fix needed — the message will be forwarded automatically. To avoid relaying, send directly to the address provided in the server's response.
Diferencias clave
SMTP 235: The client has been successfully authenticated using the AUTH command. The server will now accept mail commands from this authenticated session.
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.
You encounter 235 when after submitting valid credentials via the AUTH command (LOGIN, PLAIN, or other SASL mechanism). The server has verified your identity and you can now 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.
Cuándo usar cada uno
For 235 (Authentication Successful): No fix needed — authentication succeeded. If you expected this but received a different code, double-check your username, password, and the authentication mechanism. 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.