MrLucifer- Yes you can run a linux server in front of your exchange server to filter mail for you.


The best applications that I am aware of to block spam on a unix(linux) based system are spamassassin or perlMX.
http://au2.spamassassin.org/index.html

PerlMX runs ontop of sendmail. Here is an article that describes it in detail-http://www.perl.com/pub/a/2001/10/10/perlmx.html

All of the mail filtering packages for Exchange require a license to be purchased. If you were going to go the route of putting a 3rd party service on top of exchange, i would recommend upgrading to exchange2000 before putting any 3rd party filtering software on the server. Exchange2k has major improvements over 5.5 in 3rd party filtering functionality.

So basically what you will need to do is change your inbound DNS MX records to point to your new linux server instead of the IMC connector on the Exchange5.5 server. On the linux server I would recommend setting sendmail up to only receive mail at the domains that you host(no relay). Then hardcode the information into sendmail about your exchange5.5 imc in your hosts file. You want to make sure that the linux server is not doing a DNS lookup for your domain as your linux server is now going to be the MX host for your domain. There is a lot of good information on the web about how to configure sendmail as a relay host.

On your exchange5.5 IMC you specify that it should relay all email back out through your new linux server.

The main reason that you want to route the mail back out through the linux server is that you want to have the least amount of services visible to the outside world as possible. I'm guessing that you have a firewall, and right now your exchange5.5 IMC is sitting in the DMZ with access to port25 open. You would want to make sure that you put exchange totally behind the firewall and only allow the linux server to be in the DMZ with port25 access.

This should get you started- http://www.akadia.com/services/sendmail_relay.html