hi Ricky,
Please check the below articles for best ans.
https://www-secure.symantec.com/connect/videos/using-symantec-data-loss-prevention-block-specific-email-address-or-domain
https://www.youtube.com/watch?v=yaoVKbTf7K0
https://www-secure.symantec.com/connect/forums/blocking-specific-email-domain-content-filtering