Postfix consists of numerous services, commandos and queues. The documentation describes them in detail, but a general overview was missing – so I gave it a try.
The Postfix documentation is very detailed and of high quality. One example is postfix.org/OVERVIEW.html which explains the various services, queues and commandos and how they interact with each other. However, despite the included ASCII diagrams a “big picture” is missing. Since we do have quite some customers using Postfix at credativ my company gave me some time to spend at creating such an overview. Here you go:
The image links to the company website where you can download the PDF, but there is also a Postfix Architecture Overview Github repository including the odg, a pdf and a png.
Of course Postfix is far too complex to highlight all services and all commandos and all ways of communication, so the picture had to be simplified. For example, the process of flushing mails does only cover the deferred and incoming queue and not the processes which actually trigger the flushing. Also, policyd’s are missing, and I would actually like to include where Milter filters can be attached. Maybe I’ll update the graphic in the future. The license will be CC-BY-SA, so if you have ideas to extend the picture further you will be able to send me pull requests. Speaking about ideas, thanks to Patrick Ben Koetter who gave me the idea to create such an image.
/home/liquidat 
I posted a link to this post of postfix-users and got this reply from one of the Postfix developers:
http://archives.neohapsis.com/archives/postfix/2014-02/0511.html
FYI.
Hej Scott, thanks for the remark, I have added them as issues to the Github repo and will fix these at some point in the future =)
Hi – useful article, but the link to see the diagram doesn’t work any more as of 2014-04-04; it now lands in Credative’s main page.
Hej Davinken, thanks for pointing out. I fixed the link, it should work prorperly again =)