Mark> I'd like to configure Postfix such that I can prevent certain
Mark> IP's/networks from sending email to 'external' recipients. I'm
Mark> basically trying to set it so that our dev and test web
Mark> application servers can't email any domains other than our own -
Mark> so developers can test email functionality without the risk of
Mark> sending email out to 'real' addresses by mistake.
Set up an DNS re-direct on the dev/test subnets to another postfix
server which is only configured to forward all email to the test/dev
people. Put in firewall rules to block all other SMTP outgoing
traffic from hitting the internet.
There are other postfix solutions as well, but I'd also seriously
think about the firewall too, belt and suspenders approach.