qmail-smtpd-chkusr is a patch for qmail-smtpd (with vpopmail 4.9.x) and permits rejecting non existing users/aliases/mailing-lists in the smtp acceptance phase. I've just updaded this patch, with the following changes: - changed the error code emission (550 instead of 553) - added check for BOUNCE_MAIL flag This patch is freely available at http://www.interazioni.it/qmail