How to prevent Postfix from sending emails to certain domains
 

I set up an postfix email server for internal communication purposes only and I am able now to send email back and forth between our two internal email servers. So my question now is how can I prevent postfix from emailing to for example gmail.com, yahoo.com, aol.com and other mail domains?

Thank you for any help.

http://www.postfix.org/postconf.5.ht...t_restrictions

check_recipient_access type:table
Search the specified access(5) database for the resolved RCPT TO address, domain, parent domains, or localpart@, and execute the corresponding action