On Mon, Feb 16, 2004 at 05:57:29AM -0500, matthew wrote:
> On Mon, 16 Feb 2004, Paul A. Hoadley wrote:
> > On Mon, Feb 16, 2004 at 04:44:25AM -0500, matthew wrote:
> >
> > > > I recommend bogofilter for per-user filtering.  Spamassassin is
> > > > also highly recommended for site use.  I tend to dislike DNS-based
> > > > filtering because it has a high rate of false positives, and it
> > > > causes your users to lose legitimate mail if it's rejected at the
> > > > mail server.
> > >
> > > As far as I understand it, one does "not lose email" using dns-based
> > > blacklists.
> >
> > Sure you can.  If Alice wants to legitimately contact Bob from a
> > blacklisted IP (whether the blacklisting is actually Alice's fault, or
> > she's just fallen under an excessively large blanket), and Bob is
> > running DNS-based filtering, Bob's MTA blocks Alice based on her IP.
> > Bob loses legitimate mail.
> We have different opinions on what it means to lose email.


> An email is lost when no error message is returned to the sender and
> the email never gets to its intended recipient.

The latter may occur whether the former occurs or not.  Charles wants
to email Bob about something that's, say, important to Bob but not
that important to Charles.  Bob's MTA has Charles as blacklisted.
Let's imagine that Charles gets a bounce notification, but it doesn't
reach his threshold for doing anything more about it.  Bob loses
legitimate mail.

> So Alice knows the email was not lost.

Sure.  But the intended recipient doesn't have any information at all.

> She is now aware of why, hence the http://url in the error mesg. And
> now Alice can contact her admin, and figure out why that ip/block is
> spewing spam at me/us/blacklist users.

Sure.  Alice _could_ do that.  But if she doesn't, Bob loses
legitimate mail.

> So in summary, dns blacklists do not "lose" email.  The email was
> never sent by Alice's email server and she is aware why.

Bob didn't receive the mail.  He isn't aware why.  Bob loses
legitimate mail.


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