The email asset(s) need to be removed from any Smart Campaigns, Smart Lists, or Engagement Programs that are using them. You can see where the email is being used, by going directly to the email and once the center portion of the screen refrshes to show you the details of the email asset, down towards the bottom there will be a section called "Used By" and it will display all the smart campaigns, smart list, etc... that are using that email.