I have a qmail server relaying mail to another server that is using LDAP. The qmail server is acting as a backup MX server. What can be done about messages received by qmail but rejected upon relay to the main server due to an invalid mailbox? Obviously the spam messages have invalid return addresses which leave them in the qmail queue and waste resources via DNS lookups. Ideally they would be rejected and never queued but the qmail server is set up to accept all mail for the domain (except RBL IP's) when the main server is off-line. Any ideas other than qmail-ldap, reverse DNS lookups, or user whitelists?