I found that qpsmtpd-async can not deliver mail with queue/postfix-queue due to lack of permissions, the qpsmtpd-async process did not belong to the postdrop group.
Here is a patch to update qpsmtpd-async to set the supplementary UNIX groups, like qpsmtpd-forkserver.
--- qpsmtpd-async.orig 2008-01-30 20:41:24.000000000 +0200 +++ qpsmtpd-async 2008-02-28 22:52:52.000000000 +0200 @@ -255,7 +255,15 @@ # Drop priviledges my (undef, undef, $quid, $qgid) = getpwnam $USER or die "unable to determine uid/gid for $USER\n"; - $) = ""; + my $groups = "$qgid $qgid"; + while (my (undef, undef, $gid, $members) = getgrent) { + my @m = split(/ /, $members); + if (grep { $_ eq $USER } @m) { + $groups .= " $gid"; + } + } + endgrent; + $) = $groups; POSIX::setgid($qgid) or die "unable to change gid: $!\n"; POSIX::setuid($quid) or