I recently attempted to reconfigure an existing single-instance Postfix server (multi-homed) to support multiple MTA instances, but failed miserably (read: open relay, nastygram from ISP).
It appears that there are several methods that can be used to accomplish this, and the various (and numerous) secondary sources that I found online were 'all over the map' with respect to which they used/glued together.
Can anyone provide a working configuration (or tips) that use postmulti to manage a multi-instance Postfix setup?
Sure look here: http://www.postfix.org/MULTI_INSTANCE_README.html
The biggest point is: Multiple instances are like one instance. So if you have an open relay issue in one of the other instances then you mis-configured Postfix. Configure it like the first (normal) one and you are fine. Also note that each instance is absolutely independent from the others. If you don't want that, then multiple instances are not the solution for your problem. You should then configure one instance with multiple transports, domains, mailroutes, ports, IPs and so on.