Hi There,
I am a UK Based IT Consultant, I am used to doing projects like this and providing ongoing support for this, ASSPX is a good product and something that I have installed many times, I can do this for you today and have it running in quite a quick time.
Basically all your looking for is Spam Gateways - this is something I have done in the past (I used to use a couple of small boxes for it until it became too hefty to work with so I upgraded them).
If you have any questions then please don't hesitate to contact me, I am available on here, and if the job is awarded to me, I can also share my Skype ID with you.
John