How to configure amavisd-new for only scanning on particular senders/servers?

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http://serverfault.com – I'd like to know how to configure amavisd-new to only scan for Spam on particular clients (IPs, CIDRs or hostnames) or alternatively sender's email domain. I know that it is possible to do it on a recipient's mail address but not on how to do it for the sender's mail address. It is even possible to do it on a recipient's IP address with policy banks. But my approach should be to be independent of recipient and only relay on the sender. What I want to accomplish is to only scan mails originating from Yahoo, Google, Hotmail and the other big senders. So it is easier to configure which senders (HowTos)