Yao Ziyuan wrote: > Passive Spam Revocation (PSR) > > Currently almost all mail systems (e.g. Hotmail and Gmail) use a spam > filter, which can drop good and important messages. > > I propose an optional feature for current mail systems. The main idea > is if a message is considered spam, this spam status can be tracked by > the sender (but not sent to him directly, as the From field can be > faked). The message can be re-marked as "not spam" if the sender can > solve a CAPTCHA.
How does this differ from challenge-response filtering? http://en.wikipedia.org/wiki/Challenge-response_spam_filtering Specifically, this is a problem with forged senders; specifically, your challenge emails are themselves *spam* (see the Criticisms section of that above link). If you're proposing this as a self-imposed restriction on outbound mail from webmail providers, it might work, or it might alienate users. Most spammers don't send through webmail interfaces (even if they appear to), and sender verification tools like DKIM can defeat forged messages pretty easily already. Additionally, GMail and Hotmail aren't exactly ignoring their own users' outbound mail; spammers using accounts with GMail and Hotmail are shut down rather quickly. Any smaller deployment would simply lose users if they were to make sending mail more difficult.
