How to use smtpd_reject_unlisted_recipient?

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How to use smtpd_reject_unlisted_recipient?
\ Ken Morley (12 Jun 2007)
. \ mouss (12 Jun 2007)
. \ berny (12 Jun 2007)
. \ berny (12 Jun 2007)
. \ Ken Morley (13 Jun 2007)
. . \ Sandy Drobic (13 Jun 2007)

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Subject:	How to use smtpd_reject_unlisted_recipient?
Group:	Postfix-users
From:	Ken Morley
Date:	12 Jun 2007

I'm using Postfix 2.4.3 with Postgrey 1.27 on a mail filter box that
front-ends an Exchange server.

The mail filter has only a couple of local recipients (root, postmaster,
etc.) and mail for all others is relayed to the Exchange server. There
is a valid relay_recipients table that's automatically populated from
Active Directory using LDAP.

The problem is that we encounter dozens of dictionary attacks each day
where the spammers attempt to e-mail just about every given name at our
domain. We use tarpitting, so the spammers are disconnected after a
number of wrong guesses. Unfortunately, Postgrey is adding triplets to
it's database for each of the attempted recipients and I would like to
avoid this overhead.

Here's the smtpd_recipient_restrictions from main.cf:

smtpd_recipient_restrictions =
permit_mynetworks,
reject_unauth_destination,
reject_non_fqdn_recipient
check_policy_service inet:127.0.0.1:10023

How can I configure Postfix to reject attempts to bad recipients based
upon the relay_recipients table before calling Postgrey? I read the
documentation regarding reject_unlisted_recipient and
smtpd_reject_unlisted_recipient, but don't see how to make these work
with a table like /etc/postfix/relay_recipients.

Thanks in advance...

Ken Morley