[Exim] Putting EXIM in Front Of Exchange

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I am very interested in learning the best case practices of putting and EXIM
mail server in front of a M$ exchange server to serve as a intermediary to
do for Exchange what it cannot do for itself...

Issues of Specific Interest are:

REVERSE DNS LOOKUP
RBL , ORBS, CHECKING
pluses are SPAM BLOCKING (SPAM ASSASIN like TOOLS) and ANTI-VIRUS Tools

Most of our M$ exchange sites are installed as follows...

192.168.1.1 is the Internet Router (Linux Redhat 7.2 using Iptables for
firewall) We plan to install EXIM 3.x on these boxes to server as
go-betweens....

the Internal Exchange Servers are 192.168.1.5 on all LANS... (both Exchange
5.5 and 2000)

I would like to hear some comments or suggestions or reference sites to
making this a reality.

Andrew
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