I've developed a patch for qmail-smtpd with vpopmail, for rejecting non
existing users/aliases/mailing lists.
This patch is now working nice on my system since more than 2 months, so I
think it's pretty stable and I may submit it to the community.
The patch is freely available to everyone want to use it.
The address of the HTML page for this patch is:
http://www.interazioni.it/qmail
