I added greylisting to my IPv6 SMTP server runnung Postfix by adding the package postgrey. All incoming messages will be rejected and if the connecting sources are legitimate email servers, the messages will be queued up for retry. After the greylisted period of 5 minutes, the messages from the same sources will then be accepted by postgrey. In the case of spam emails by zombie computers, the zombies which do not act like a SMTP server, will not store and queue up emails for subsequent delivery. On the whole, I believe greylisting is over 90 % effective to reject spam from zombies.