SMTP 251 User Not Local vs 500 Syntax Error
SMTP 251 (User Not Local) is a 2xx Positive Completion response, while 500 (Syntax Error) 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, 500 means that the server could not recognize the command due to a syntax error. The command line was malformed or the command is not recognized by this server.
Descrição
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.
Quando você o vê
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.
Como corrigir
No fix needed — the message will be forwarded automatically. To avoid relaying, send directly to the address provided in the server's response.
Descrição
The server could not recognize the command due to a syntax error. The command line was malformed or the command is not recognized by this server.
Quando você o vê
When you send a command the server does not understand — often due to a typo, unsupported command, or an excessively long line.
Como corrigir
Check the command spelling and syntax against the SMTP specification. Ensure the command line does not exceed 512 characters and uses proper CRLF line endings.
Diferenças principais
251 is a 2xx Positive Completion response, while 500 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 500: The server could not recognize the command due to a syntax error. The command line was malformed or the command is not recognized by this 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 500 when when you send a command the server does not understand — often due to a typo, unsupported command, or an excessively long line.
Quando usar qual
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 500 (Syntax Error): Check the command spelling and syntax against the SMTP specification. Ensure the command line does not exceed 512 characters and uses proper CRLF line endings.