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.

Fig. 1 The usual setup

Fig. 2 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' $max_idle/ $max_use or $transport_time_limit.