I don't know how to fix this one, cos I don't know the ins and outs of perl UID handling. But qpsmtpd-forkserver doesn't drop its GID correctly:
2005-02-25 11:57:11.809180500 24306 Running as user smtpd, group root
I'm not seeing that with v5.8.3, so I suspect that may be a 5.6.x problem. Indeed there seem to have been a couple of changes to the POSIX::setuid/gid code listed in Changes5.8.
I'd recommend upgrading if you can...
John
