A very simplified scheme of how policyd-weight can reduce CPU, MEM and
bandwidth-usage on a mailer.
Keep in mind, that this won't work for POP3 (fetchmail and others) users.
The usual setup
Setup with policyd-weight
The SMTPD asks the policy daemon at RCPT TO stage and waits for the result.
The result is cached with a IP/Sender key to minimize DNS lookups in the policy
daemon itself with a lifetime of postfix'