Which, in spite of its 'perfection', it should be noted that you should not run unpatched on a public facing server (or at least not qmail-smtpd), lest you become an unwitting backscatter zombie.
I love djb's software, and I love his way of looking at things, but no software stays perfect forever.
Had to look up what a backscatter zombie was; apparently it's what happens when you accept bogus mail (spam) into your mail queue without checking if it is actually deliverable -- thus end up bouncing it once the queue is processed for delivery -- and typically then sending it back to a forged address -- ie: you become a spam gateway (spammers send you email with sender-addresses they want you to try to send spam to...):
https://github.com/amery/qmail/tree/aa6bf9739209ca76f7f3af0f...