I wouldn't call the designs of Qmail or Postfix "strange", they're very well designed systems for what they do and given their age. I intimately know the workings of postfix, and while it's not nginx, it does everything it can to be modular and extensible: see how many ways it allows you to access the mail queue before a message is delivered, for example. Yep, THREE different, well documented ways, in increasing levels of ease/performance.

P.S. Is it just me or has Exim declined in popularity? 10 years ago it was hard to choose between Qmail and Exim, and Postfix was the hot new thing.