Fortunately using greylisting with Postfix is rather trivial. You would need to install the postgrey package first:
$> aptitude install postgrey
Right after its installation the postgrey daemon will start running in the background and listen for requests on TCP port 60000. To make Postfix use that policy daemon you have to add it to the smtpd_client_restrictions setting in your /etc/postfix/main.cf. Example:
smtpd_client_restrictions =
permit_mynetworks
permit_sasl_authenticated
reject_unauth_pipelining
reject_rbl_client bl.spamcop.net
reject_rbl_client dynablock.njabl.org
reject_rbl_client zen.spamhaus.org
reject_rbl_client list.dsbl.org
check_policy_service inet:127.0.0.1:60000