|Anti-spam technique: Captchas| |Date of first use:||ca. 2001|
|Difficulty of implementation:||Medium| |Where implemented:||MTA
or MUA| Captchas (from "Completely Automated Public Turing test
to tell Computers and Humans Apart") is version of mail
challenges, adapted from a method originally designed to protect
web forms from being completed by robots. When using to protect
a mailbox, a confirmation message is sent to the sender when he
writes to the protected mailbox for the first time, indicating a
web page where he will be asked to enter a textual code hidden
in an image. If the correct answer is given, the sender is added
to a whitelist so subsequent messages will be accepted without
any further action. Although some defenders of this method claim
it provides "100 % spam stopped without loss of messages", in
practice this method has many drawbacks including: - All of the
general problems with Challenges - Not user friendly with people
with physical deficiencies - blind people may not be able to
correctly