SMTP 251 User Not Local vs 554 Transaction Failed
SMTP 251 (User Not Local) is a 2xx Positive Completion response, while 554 (Transaction 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, 554 means that the mail transaction has failed. This is a catch-all permanent error that can occur at any point during the SMTP session, including as a rejection greeting when the server refuses all connections from
Mô tả
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.
Khi bạn thấy mã này
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.
Cách khắc phục
No fix needed — the message will be forwarded automatically. To avoid relaying, send directly to the address provided in the server's response.
Mô tả
The mail transaction has failed. This is a catch-all permanent error that can occur at any point during the SMTP session, including as a rejection greeting when the server refuses all connections from your IP.
Khi bạn thấy mã này
When the entire mail transaction is rejected — often due to spam filtering, blacklisted IP addresses, malformed messages, or policy violations detected during the DATA phase.
Cách khắc phục
Check your IP against DNS blacklists (DNSBL) and verify your SPF, DKIM, and DMARC records. Review the full error message for specific rejection reasons. If blacklisted, request delisting from the relevant blocklist.
Sự khác biệt chính
251 is a 2xx Positive Completion response, while 554 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 554: The mail transaction has failed. This is a catch-all permanent error that can occur at any point during the SMTP session, including as a rejection greeting when the server refuses all connections from
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 554 when when the entire mail transaction is rejected — often due to spam filtering, blacklisted IP addresses, malformed messages, or policy violations detected during the DATA phase.
Khi nào dùng cái nào
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 554 (Transaction Failed): Check your IP against DNS blacklists (DNSBL) and verify your SPF, DKIM, and DMARC records. Review the full error message for specific rejection reasons. If blacklisted, request delisting from the relevant blocklist.