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<h3>TMDA's Challenge/Response system</h3>
Challenge/Response is an old idea, and one which has been
used by mailing list management software for years, but it's
surprisingly effective against spam.<br><br>
<u>TMDA's Whitelist-centric Strategy</u>
<strong>``Deny everything that is not <em>explicitly allowed</em>''</strong><br>
<ul>
With TMDA, unrestricted access to your mailbox can no longer be
assumed, a premise which spammers rely heavily upon.<br><br>
The way TMDA thwarts incoming junk-mail is simple yet extremely
effective. You maintain a "whitelist" of trusted contacts which are
allowed directly into your mailbox. Messages from unknown senders are
held in a pending queue until they respond to a one-time confirmation request
or "challenge" sent by TMDA. Once they respond to the confirmation, their original
message is deemed legitimate and is delivered to you. TMDA then adds
their address to your whitelist so they won't have to confirm future messages.
To see what the confirmation process looks like,
<a href="mailto:tmdatest@tmda.net">send me a test message</a>,
and then reply to the confirmation request.
<br><br>
This methodology has the advantage of being very selective about what
it allows in, while at the same time permitting legitimate, but
previously unknown senders to reach you.
<br><br>
Optional use of TMDA's <a href="config-client.html">tagged addresses</a>
will greatly reduce the number of unknown senders who are actually sent a
confirmation request.
</ul>
<u>Traditional Blacklist-centric Strategy</u>
<strong>``Allow everything that is not <em>explicitly denied</em>''</strong><br>
<ul>
Traditional anti-spam technical countermeasures are based upon
maintaining a "blacklist" containing e-mail addresses, domains,
and/or network subnets of known junk-mailers. Or worse, a "profile" of
message headers and message body text that fits the software's idea of
what a piece of spam looks like.<br><br>
The problem with this approach is that spammer's intrusion techniques
are evolving as fast as your prevention techniques are, so the battle
is never ending. Maintaining the blacklist or spam "corpus" is often just as
time-consuming as pressing the `Delete' key on the easily recognized
junk messages. If wasted time is your biggest complaint with junk
e-mail, you can see why these traditional methodologies are flawed.
<br><br>
The chance of accidental "false positives" is also significantly
higher with this more complex approach. If you really want effective
and reliable UCE control, you need something like TMDA that doesn't rely on
heuristics that spammers can work around.
</ul>
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