There are some email servers (which I don't know) that automatically
can send back an email to the sender,
asking the sender to authenticate himself by clicking on a link in the
message or by simply replying to the
message.
Ah, it sounds like someone wanted me to post again! :)
Top 10 reasons why challenge/response (C/R) is bad:
[1] You end up being a spammer (the majority of spam sent to you will
result in confirmation requests being sent to innocent victims)
[2] Spammers now send pretend confirmation requests, presumably to make
people less likely to respond to C/R requests
[3] Many people respond to C/R requests that they never initiated
(sometimes intentionally, sometimes not). Some people who are fed up
with bogus C/R requests respond to all of 'em, knowing that the spam
will start getting through to people hiding behind C/R.
[4] C/R companies have been known to send out spam and harvest addresses
of people sending to their customers, and apparently sell those
addresses to spammers
[5] The C/R system is patented, so most anti-spam programs using C/R
have legal liabilities waiting to be ironed out. The C/R program you
buy today may go under tomorrow.
[6] Confirmations sent to mailing lists won't work
[7] Confirmations sent to others using C/R won't work. If everybody had
C/R, nobody could send E-mail to anybody!
[8] People who offer a free service end up losing money (by spending
time investigating and responding to C/R systems, dealing with spam
received as a result, etc.) and sometimes get fed up with C/R systems
and eventually stop offering free advice (never knowing how many people
won't get their E-mails), harming everybody.
[9] Legitimate E-mail from automated services won't be seen (such as
when ordering products online)
[10] Due to #1-#9, most C/R challenges are treated as spam -- if the
challenge never gets through, the response will never get through.
-Scott
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