greylisting with postgrey

Debian and Ubuntu have the package available in the main repositories. On CentOS/RHEL based distros, you will need to enable the rpmforge repository per http://bliki.rimuhosting.com/space/retep/extra+packages+for+rhel4+and+centos5

Then just install postgrey with apt

apt-get install postgrey

Configure postgrey to start at boot:

Debian/Ubuntu:

update-rc.d postgrey defaults

CentOS/RHEL:

chkconfig postgrey on

And tell it to start up (any distro):

/etc/init.d/postgrey start

Now, tell postfix to use it:

Debian/Ubuntu: The package listens on localhost:60000 by default, so edit /etc/postfix/main.cf, and add 'check_policy_service inet:127.0.0.1:60000' to smtpd_recipient_restrictions. Make sure to add it after permit_sasl_authenticated so you don't greylist authenticated users. Like this:

smtpd_recipient_restrictions =
        permit_mynetworks
        permit_sasl_authenticated
        reject_unauth_destination
        check_policy_service inet:127.0.0.1:60000

CentOS/RHEL: This package listens on a unix socket by default, so edit /etc/postfix/main.cf and add 'check_policy_service unix:/var/spool/postfix/postgrey/socket' to 'smtpd_recipient_restrictions. Again, make sure to add it after permit_sasl_authenticated so as not to greylist yourself. Example:

smtpd_recipient_restrictions =
        permit_mynetworks
        permit_sasl_authenticated
        reject_unauth_destination
        check_policy_service unix:/var/spool/postfix/postgrey/socket