Exim filter not working with email forward

I have server mail with forward to a yahoo email, but I wan't send email forward if email have header from domain is "@yahoo.com". Begin i try will block all email forwarder to email "someusr@yahoo.com" but it filter not working, Emails still forwarder to email "someusr@yahoo.com", My Filter: if $header_to: contains "someusr@yahoo.com" then fail text "Your message to inform email users about the problem..." finish endif if not first_delivery then finish endif
Please help me ! Thanks