We've got qmail/vpopmail running fine with the latest chkusr patch from Bill's page (http://www.shupp.org/).
Our backup mxer has been running sendmail for ages, and we'd like to move it to qmail. We'd also like put vpopmail on there just so we can reject bogus addresses on there as well rather than queuing them and having them bang on the main mxer each time the queue runs.
I've got qmail and vpopmail compiled there. Started with netqmail and then Bill's big jumbo patch and the chkusr patch. There's a daily rsync of the ~vpopmail/domains tree (.qmail-* only) to the backup mxer. We're testing on an alternate port (26). Everything works fine except chkusr is not blocking anything; I can enter a totally bogus address and it's accepted.