I needed to add some more ‘security’ to our mail chain, which required
adding more checking on the MX Proxy, which is our host that sits on the
Internet ‘proxying’ mail traffic in and out of the organisation.
To improve our security profile, using our preferred Postfix, necessitates
incoming mail be processed independently of outgoing mail.
To save money on a 2nd computer hardware and using another Public IP address,
we used Postfix’s Multiple Instance facilities, as well as using separate
ports for internal mail and external mail.