Mail server greylisting

Folks:

For a while I've been using limited greylisting on my mail
server with
reasonably good success.

Last weekend I implemented site wide and I have to say the
results are
dramatic.  The amount of spam (even low rated by
spamassassin) has
dropped off significantly.

Detailed information on greylisting can be found at
http://pro
jects.puremagic.com/greylisting/, but in a nutshell:

Greylisting relies on the fact that spammers don't use
normal mail
servers.  Basically, the first time a mail server receives a
mail
delivery request, it responds with a soft failure ... with a
message
indicating that greylisting is in effect and they should
retry the
delivery in certain amount of time (this is a human readable
message,
not machine readable).  Since normal mail servers will
accept this
message and requeue the message for delivery, mail will be
delivered
normally (probably on the next pass).

Spammers aren't that persistent, so they just go on to
their next target.

A good greylisting implementation retains the list of
servers that have
successfully delivered in a whitelist, so the next time they
try to
deliver there is no delay, the delay is only encountered
once.

I've got my mail server configured to greylist servers for
only 2
minutes ... so the next time the server tries to deliver,
it's almost
certain to be successful the next time.

I'm using Milter-greylist (http://hcpnet.
free.fr/milter-greylist/) with
sendmail.  It was easy to setup and works great.

david
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David Gibbs wrote:
> Greylisting relies on the fact that spammers don't use
normal mail
> servers.  

One downside of greylisting that I've found is that there
is an
increased chance of messages arriving out of order when a
server tries
it's first message deliver.

The reason is this ... the first message delivery will be
attempted and
be rejected due to greylisting, if a different message
delivery is
attempted AFTER the greylisting delay has expired but BEFORE
the first
message is delivered, then the second message will be
delivered and will
be out of context.

david
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