SMTP 252 Cannot Verify User vs 503 Bad Sequence
SMTP 252 (Cannot Verify User) is a 2xx Positive Completion response, while 503 (Bad Sequence) is a 5xx Permanent Negative response. 252 indicates that the server cannot verify the user but will accept the message and attempt delivery. This is often returned in response to VRFY when the server intentionally hides user information. In contrast, 503 means that the commands were sent in the wrong order. SMTP requires a specific command sequence (EHLO, MAIL FROM, RCPT TO, DATA) and this error indicates a step was skipped or repeated.
描述
The server cannot verify the user but will accept the message and attempt delivery. This is often returned in response to VRFY when the server intentionally hides user information.
何时出现
When using the VRFY command to check if a recipient exists. The server refuses to confirm or deny the address, typically as a spam-prevention measure.
如何修复
No fix needed — the server will still attempt delivery. If you need to verify addresses, contact the mail administrator or rely on bounce-back messages instead.
描述
The commands were sent in the wrong order. SMTP requires a specific command sequence (EHLO, MAIL FROM, RCPT TO, DATA) and this error indicates a step was skipped or repeated.
何时出现
When sending DATA before RCPT TO, or MAIL FROM before EHLO. The server enforces the correct SMTP command sequence and you have sent a command out of order.
如何修复
Follow the proper SMTP command sequence: EHLO/HELO first, then MAIL FROM, then RCPT TO (one or more), then DATA. Reset with RSET if you need to start over.
主要区别
252 is a 2xx Positive Completion response, while 503 is a 5xx Permanent Negative response.
SMTP 252: The server cannot verify the user but will accept the message and attempt delivery. This is often returned in response to VRFY when the server intentionally hides user information.
SMTP 503: The commands were sent in the wrong order. SMTP requires a specific command sequence (EHLO, MAIL FROM, RCPT TO, DATA) and this error indicates a step was skipped or repeated.
You encounter 252 when when using the VRFY command to check if a recipient exists. The server refuses to confirm or deny the address, typically as a spam-prevention measure.
You encounter 503 when when sending DATA before RCPT TO, or MAIL FROM before EHLO. The server enforces the correct SMTP command sequence and you have sent a command out of order.
何时使用哪个
For 252 (Cannot Verify User): No fix needed — the server will still attempt delivery. If you need to verify addresses, contact the mail administrator or rely on bounce-back messages instead. For 503 (Bad Sequence): Follow the proper SMTP command sequence: EHLO/HELO first, then MAIL FROM, then RCPT TO (one or more), then DATA. Reset with RSET if you need to start over.