SMTP 251 User Not Local vs 521 Host Does Not Accept Mail
SMTP 251 (User Not Local) is a 2xx Positive Completion response, while 521 (Host Does Not Accept Mail) 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, 521 means that the host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
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 host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
このコードが表示される場合
When connecting to a server that has been intentionally configured to refuse all email, such as a domain that only sends but never receives mail.
解決方法
Verify you are connecting to the correct mail server by checking the domain's MX records. If the domain truly does not accept mail, contact the recipient through an alternative channel.
主な違い
251 is a 2xx Positive Completion response, while 521 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 521: The host does not accept mail at all. This indicates the server exists but is explicitly configured to reject all incoming email messages.
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 521 when when connecting to a server that has been intentionally configured to refuse all email, such as a domain that only sends but never receives mail.
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 521 (Host Does Not Accept Mail): Verify you are connecting to the correct mail server by checking the domain's MX records. If the domain truly does not accept mail, contact the recipient through an alternative channel.