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://projects.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