Server :: Rejecting Mail Where Sender Matches Recipient In Postfix?

This seems like a relatively simple question, but I haven't been able to figure it out from the documentation after a couple of hours of searching. I'm running postfix on my mail server, and the vast majority of my spam has the sender address spoofed to match the recipient address. I've got spamassassin up and running, and very little gets through that, so it's not a serious problem, but if possible I would like to be able to reject the mail before it arrives to reduce the system load.

Mail is sent locally via webmail, relaying is denied, so the only way that mail should be sent from the domain is via a connection from localhost. The basic idea I'm thinking of is if the MAIL FROM sender claims to be an address at the receiving domain, but is connected from a remote IP, the mail should be rejected.