To further reduce spam, consider this stanza in main.cf:
smtpd_sender_restrictions =
       	permit_mynetworks,
       	permit_sasl_authenticated,
       	check_sender_access pcre:/etc/postfix/reject_domains hash:/etc/postfix/sender_access,
       	reject_unknown_sender_domain,
       	reject_unknown_reverse_client_hostname,
       	reject_unknown_client_hostname