This was sitting in a tab in Safari. I don't remember how it got there. It is a pdf of an Apollo document from 1985. Maybe it was from reading Steve Vinoski's latest RPC, etc. stuff - Steve worked at Apollo.
Maybe you're interested in reading it, maybe not. Apollo workstations were the first true "the network is the computer" computer, predating SunOS. I used them at Boeing and at Mentor Graphics in the 80's and 90's.
The CAD tools I worked on at Mentor Graphics used mailboxes to communicate between components, e.g. back end simulation engines and front end graphics. Note that Pascal is the systems programming language for Apollo.