Воткните следующие строчки в main.cf

smtpd_helo_required = yes
disable_vrfy_command = yes
strict_rfc821_envelopes = yes
smtpd_client_restrictions =     reject_rbl_client blackholes.mail-abuse.org,
                                reject_rbl_client dialups.mail-abuse.org,
                                reject_rbl_client dul.ru,
                                reject_rbl_client list.dsbl.org,
                                reject_rbl_client sbl.spamhaus.org,
                                reject_rbl_client cbl.abuseat.org,
                                reject_rbl_client dul.dnsbl.sorbs.net,
                                reject_rbl_client dnsbl.njabl.org,
                                reject_rbl_client dynablock.njabl.org,
                                reject_rbl_client combined.njabl.org,
                                permit_mynetworks,
                                reject_unknown_client,
                                permit

В неделю на мою почту приходит около 2-5 спам-писем о рассылке на почтовые ящики :)