Relaying only mail server && spamassasin

[nevynxxx]

I have an idea bubbling in my head. My company has exchange 5.5 on win NT4 (ewww yes, well i'm trying to change it slowly). I would like to have something to stop spam. My idea is this, plop postfix (as I already use postfix so I know it better than anything else) between exchange and the internet. So exchange uses postfix as an smpt relay for outgoing mail, and postfix gets all the incoming mail and farwards it on to exchange. I know this much is possible. The part where I'm having trouble figuring it out is spamassasin.

Would I be able to get postfix to launch spamassasin, filter out the spam then farward whats left onto exchange? This would be a very good way of justifying our first linux server, and also the first step on the move to taking exchange out.

Any ideas, comments critisisms?
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[cogitate]

This works quite well. I'm working on something similar right now. I don't have any documentation to help you with, but the spamassassin site and the postfix site both help in getting this working.

If you check out this mailhosting howto for the spamassassin setup. https://forums.gentoo.org/viewtopic.php?t=56633&highlight=mailhost