Esto es parte del archivo de configuracion de mi Postfix (main.cf) en cual en gran medida me ayudado a evitar gran cantidad de Spam, espero les sea de utilidad:
smtpd_sasl_auth_enable = yes
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_helo_required = yes
smtpd_helo_restrictions = reject_invalid_hostname
smtpd_recipient_restrictions =
permit_sasl_authenticated,
permit_mynetworks,
reject_unauth_destination,
reject_unauth_pipelining,
reject_non_fqdn_recipient,
reject_non_fqdn_sender,
reject_unknown_recipient_domain,
reject_unknown_sender_domain,
check_recipient_access pcre:/etc/postfix/recipient_checks.pcre,
check_helo_access pcre:/etc/postfix/helo_checks.pcre,
check_client_access pcre:/etc/postfix/client_checks.pcre,
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
smtpd_sender_restrictions = reject_unknown_sender_domain
disable_vrfy_command = yes
