Both the architecture and the approach to documenting it were pioneered by Bernstein's qmail, which is the first Unix program of comparable ambition to be structured in this way --- it's crazy to think that there was a time when this implementation strategy was groundbreaking, but, it was.
(Fun fact: Venema and Bernstein had a long-running feud, and Postfix exists pretty much entirely because Venema appreciated qmail's architecture but couldn't stomach working with anything Bernstein produced.)
http://cr.yp.to/qmail/pictures.html
(Fun fact: Venema and Bernstein had a long-running feud, and Postfix exists pretty much entirely because Venema appreciated qmail's architecture but couldn't stomach working with anything Bernstein produced.)