[NTLUG:Discuss] Making a spam shield server

[. Daniel]

Here's the scenario:

At the office is a MS Exchange 2003 server.  I would like to put up a  
Linux box (got it loaded and just about ready to go) running CentOS 4.4 to 
accept mail for the domain and forward it on to the Exchange server after 
processing through greylisting and spam assassin's filters.

The problem is that I get a user unknown error.  So either I create users 
manually (not gonna happen) or come up with a way to link the server to the 
Active Directory domain via LDAP as a means of determining whether or not a 
user exists.

There is a sendmail configuration that will allow for simply accepting ALL 
mail incoming for a domain, but that's a dangerous proposition considering 
all the directory lookup bots running out there. (Ever check your mail logs 
and see attempts at delivery to
alphabetically sorted random names?  They are scanning for valid 
accounts... heaven forbid I give spammers a reason to hit my mail server 
with billions of invalid email account requests.)  Further, I get no 
indication that SpamAssassin gets its hands on the mail before passing it 
on which is kind of useless.

So anyone with any such knowledge care to share?

(What I have so far is a CentOS 4.4 box configured with relaydelay and spam 
assassin.  It seems to be working just fine and just needs configuration to 
suit the needs described above.)

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