Just ran across a CNET News.com article describing the concept, seems to be
a useful way to make sure a human is sending email:
http://news.com.com/2100-1032_3-1015247.html
Unfortunately, the article seems to be confuse this system with general
C/R. Here are some quotes:
---snip---
"But in recent weeks, people using Yahoo Mail have found themselves asked
to type in camouflaged letters before they can send an e-mail message, in
an "image verification" method. "
"Yahoo stressed that people sending mail through its system are not
required to pass a challenge-response test for every message, or even for
every mail-sending session."
"Instead, in order to keep spammers guessing, Yahoo's system computes what
the company calls a "user profile." This profile details the frequency with
which the user normally sends messages and the length of time the user has
held the account. The system issues a spam challenge when the user
approaches an individually determined rate limit."
---snip---
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Yakov Shafranovich / <research(_at_)solidmatrix(_dot_)com>
SolidMatrix Research, a division of SolidMatrix Technologies, Inc.
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"One who watches the wind will never sow, and one who keeps his eyes on
the clouds will never reap" (Ecclesiastes 11:4)
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