When someone opts to unsubscribe from one of your campaigns, that person is disabled for all future campaign mailings from Pipeline. You will be prompted with a notification if you attempt to email someone who has previously unsubscribed from any email communication. Please keep in mind that they will still receive individual emails. 

To see a list of who has unsubscribed to campaigns, you can go to the People tab, click Add/Remove columns then check off the Unsubscribed column. Once the column is displaying, you can filter the Unsubscribe by selecting yes. This will display all people on the list that have unsubscribed. Note: If a recipient unsubscribes from a previously scheduled campaign, they will still receive it for that campaign only.