Flagged emails are a result of contacts unsubscribing from a bulk email, potential spam addresses, and emails that bounce from their server.
In Crelate, navigating to Settings >>Flagged Emails will allow you to view all emails that have been flagged so you may manage each recipient in addition to being able to add contacts.
Selecting the sign will allow you to remove contacts from this list.
Selecting the sign will allow you to manage and add additional contacts to the list. (Use Case: A contact has asked to no longer be contacted but never actually unsubscribed. I may unsubscribe them manually in my database to ensure they're not included in future campaigns.) Even if the contact is accidentally included in a future email campaign, they will not be sent the email message.
You also have the ability to track and find all emails that have been flagged by going to: Contacts>>Recent Contact Drop Down>>Flagged Emails (scroll to the bottom to find).
Each type will be indicated by 3 icons: