[CentOS] spam control

> I have found that greylisting based on the Spamhaus sbl-xbl list
> massively reduces spam.  Setting that up on my backup MX
> resulted in all traffic through it stopping when my primary MX is
> up.

James,

Are you using greylisting or blacklisting with Spamhaus?  My understanding 
of blacklisting is that blacklisting uses a DNS blacklist, such as 
Spamhaus.  My understanding of greylisting is when your MTA sends back a 
temporary failure the first time in X number of days where it sees a new 
sender/receiver/mail server IP address triplicate.  Please clarify.

Thanks,
Barry