How do you set exchange 2003 to only accept incoming email on port 25 from specific ip range?

I have a new exchange 2003 server. I use a 3rd party spam filtering company. My dns is setup so email goes to the spam filtering company first, they filter the email for spam and viruses, and then shoot me only the good stuff. The problem I am having is that spammers are circumventing dns by blasting my server directly (ignoring dns) with smtp messages. Unfortunately my firewal doesn't seem to have any advanced options to allow me to configure to block smtp traffic unless from the ip range of my spam filtering company. I believe their is an option in exchange to allow one to specify an ip range that the server will allow incoming smtp from. I would then like to reject emails coming from all other ip addresses. My router is a 2wire AT&T dsl basic router.
FASTECHSAsked:
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rakeshmiglaniCommented:
there is an option in message delivery under global settings. the tab you are looking for is connection filtering
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rakeshmiglaniCommented:
the smtp virtual server also has settings that can be configured under the access tab connections button
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