[Dovecot] Emails from invalid local accounts

Hi,

I'm using dovecot 1.2.9 in a postfix/dovecot setup and I'm having issues with receiving spam where the "from" header contains an address like accounting@mydomain.com. Is there some way I can filter out these emails coming from outside our network with an account associated with our network which doesn't exist? Do I just need to configure some custom process to evaluate these addresses, or is there some way either in dovecot or spamassassin to do this?

Thanks,

Steve K