Need help to create an attachment policy in DLP to trigger those mails which user sends attachment data out side the domain

Dear Team,

   I want to create an attachment policy in DLP to trigger those mails which sends attachment data out side the domain external domain like Gmail.com or Yahoo.com etc. We don't want to block those mails only we should get an alert or notification when any user sends the mail to such ID's. 

Regards,

ABUL-Symantec Engineer

Mob:8095767906