block outgoing spam

[Infoomatic via amavis-users]

hi list,

I want to reject outgoing spam from my server.
Users, aliases and domains come from LDAP, postfix uses smtpd_sender_restrictions = reject_sender_login_mismatch

However, I did not quite figure out to handle incoming spam different from outgoing. 
i want something like
if (sendermaildomain = one_of_my_domains){
  $final_spam_destiny       = D_REJECT;
} else {
  $final_spam_destiny       = D_PASS;
}

I found various things to bypass scanning of outgoing mails, but no solution for my case. Mail is sent only via port 25, clients come from dynamic ipadresses.
Or is it possible in postfix to use different content_filter for incoming and outgoing mails? ... I could start 2 amavis instances on different ports, one handling outgoing and one handling incoming mails, however I don't know how to configure this in postfix or distinguish incoming/outgoing mails.

thanks in advance,
infoomatic