We just noticed once again, that postfix is so well designed in a way, that often we did not even think of "corner cases" that are already handled by default in a way, that is in most cases exactly how is should be setup.
Just picking a random setup - relaying mails to external relayhosts by sender domain but having the option to define individual transport ways. This is awesome to give the user/administrator a way to do a very fine grained mail routing. We're a "small" postfix uers (< 100.000 mails / month) but are very happy with postfix. Thank you. Stefan