This is EXACTLY the right solution and a mighty fine testament to the forethought the author gave to the problem.
Last night I finally activated the qpsmtpd installation I've had sitting on my server for almost a year. As I watched mail deliveries come in, I paid special attention to all the error messages and worked the many kinks with getting qpsmtpd installed and all the plugins working. Hacking the spamassassin plugin so per-user prefs work, tweaking permissions so clamav works. Bumping up the RAM because I'm running amd64. Etc, etc, etc...
After everything was working as I expected, I started getting bounces for non-existing emails. Holy antiquated problems batman! We solved this problem a decade ago with the qmail chk-user patch. And yet qpsmtpd has no solution. :-( The qpsmtpd dist has a STATUS file mentioning adding support for check_delivery, which no longer exists at the referenced URL. So I found it on archive.org. And it's a half-baked solution that requires suidperl.
I was starting to fear having to write a solution myself, and then I found this. The docs are good, the interface is good, the architecture is excellent, and it works! Thank you, thank you, thank you!