On 26/10/09 01:32, Yao Ziyuan wrote: > Currently almost all mail systems (e.g. Hotmail and Gmail) use a spam
> filter, which can drop good and important messages.
Only by flawed design. There's a 5xx SMTP response which could very
clearly indicate that it was spam.
> In the first case, A's mail client will report the status and the
> CAPTCHA to A. A can choose to solve the CAPTCHA to prove the message
> is not spam.
This only proves that a human or sophisticated program solved the CAPTCHA.
