В последнее время замучил спам вида: user@domain.com –> user@domain.com, т.е. спам, якобы, от себя – себе же.

Правим, добавляя выделенное:

# ee /usr/local/etc/postfix/main.cf
......
# Aутентификация SMTP
smtpd_sasl_auth_enable = yes
smtpd_sasl_exceptions_networks = $mynetworks
smtpd_sasl_security_options = noanonymous
broken_sasl_auth_clients = yes
smtpd_sasl_type = dovecot
smtpd_sasl_path = private/auth

# SPAM user@domen.com --> user@domen.com
smtpd_restriction_classes = OnlyFromMyUsers

OnlyFromMyUsers = permit_mynetworks
 permit_sasl_authenticated
 reject
#*** END SPAM *** access_sender ***

smtpd_client_restrictions =
 permit_mynetworks
 permit_sasl_authenticated
 reject_unauth_pipelining
 permit
....

# Ограничения по отправителю
smtpd_sender_restrictions =
 check_sender_access hash:/usr/local/etc/postfix/access_sender
 check_sender_access hash:/usr/local/etc/postfix/blacklist
 permit_mynetworks
 permit_sasl_authenticated
 check_sender_access hash:/usr/local/etc/postfix/whitelist
 reject_non_fqdn_sender
 reject_unknown_sender_domain
 permit
....

# cat access_sender
domain.com OnlyFromMyUsers

Перезапускаем сервис:

# service postfix restart