[ale] Spam filtering

[Chris Ricker]

On Sun, 13 Jul 2003, Christopher R. Curzio wrote:

> Well, since you're probably referring to addresses, you're probably asking
> about how to test the validity of the sender, and not necessarily the
> server.
> 
> Sendmail can make sure that mail is rejected from bogus host names in the
> FROM field, but this will only weed out a small amount of spam, since
> spammers have gotten wise to this and typically forge real domain names.
> 
> As for validating the SENDER, I don't know that you can do that. Check
> here for your options with sendmail: http://www.sendmail.org/antispam.html

Assuming I'm following what kind of validation is being asked about, it's
something that I think at the MTA level only Postfix and Exim can do
currently. At least in Postfix circles, it's called variously "sender
address verification" or "SMTP interruptus". The basic idea, in the Postfix
implementation, is that unknown senders are blocked until verified by
probing the unknown address (if MAIL FROM: foo, RCPT TO: unknown_address is
accepted by server, then unknown_address is legit, the probe is terminated
and unknown_address is accepted; if MAIL FROM: foo, RCPT TO:  
unknown_address is rejected by server, unknown_address is bogos, the probe
is terminated, and mail from unknown_address is rejected). Once verified,
addresses are added to a database of validated senders....

The idea itself has been around for a long time, and implemented in the past
with a bit of work on Sendmail (somewhat ironic, since I've been told by one
core Sendmail developer that it'll never appear in Sendmail proper, though
I'm not entirely convinced that we weren't talking past each other ;-). It's
also potentially encumbered by a junk patent, US Patent 6321267.

I use it. It works quite nicely, but you do have to be careful at first to 
prepopulate your whitelist database.

later,
chris
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