Postfix - how to block emails sending out to a particular domain
Solution 1:
A closer look at the basic access controls available should help you out; just specify the appropriate access map in smtpd_recipient_restrictions that denies access to those domains:
smtpd_recipient_restrictions = check_recipient_access hash:/etc/postfix/blacklisted_domains, permit_mynetworks, reject_unauth_destination, permit
and in /etc/postfix/blacklisted_domains:
bad.domain1 REJECT
bad.domain2 REJECT
[email protected] REJECT
Don't forget to postmap that file.