Te recomiendo usar MailScanner, es muy efectivo. http://www.linuxparatodos.net/portal/staticpages/index.php?page=como-mailsca...
Negativo yo te recomiendo usar la variante Postfix+Amavis-new+ClamAv+Spamassassin,
Amen que es importante que endurescas tu Postfix, por ejemplo:
mynetworks = 127.0.0.0/8, numeros.ip.de.tus.servidores relay_domains = $mydomain, midominio.com smtpd_helo_required = yes smtpd_helo_restrictions = reject_invalid_hostname reject_unknown_hostname reject_non_fqdn_hostname
smtpd_recipient_restrictions = permit_mynetworks reject_unauth_destination reject_non_fqdn_sender reject_non_fqdn_recipient reject_unknown_recipient_domain reject_unverified_recipient
smtpd_client_restrictions = permit_mynetworks, reject_rbl_client dnsbl.sorbs.net reject_rbl_client zen.spamhaus.org reject_rbl_client cbl.abuseat.org
Esto te puede servir para endurecer aun mas tu Postfix...
Suerte, David