OT: How to defeat address harvesting; like this..???

[DNSAdmin]

Hello All,

Less than 24 hours of changing over a domain to a new mail server, and I've 
got a gnat I need to swat!

|Unknown users:
|
|   arnold@******.com
|      from [222.233.140.203]    1 time(s).
|
|    bowen@******.com
|      from [222.233.140.203]    1 time(s).
|
|    boyd@******.com
|      from [222.233.140.203]    1 time(s).
|
(List condensed for mail list purposes. They tried to guess about 20 users 
last night)

This is a harvesting attack to determine valid usernames to compile a spam 
list.

How do I defeat this??? Any decent suggestions appreciated! I might even 
consider the baseball bat if he/she were in my backyard!

I have CentOS 3.4, Sendmail 8.12, MailScanner 4.41, SpamAssassin 2.55

TIA!
Glenn Parsons 


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