When you send email distributions there is a chance that a media contact will unsubscribe from receiving future emails. This can occur, for example, because the subject of the email distribution is not relevant to the contact, in their opinion.
When this occurs Agility will identify that the contact has unsubscribed, and will automatically prevent your future emails from being delivered to this contact.
There is one further step you can take.
Within Measure, you can access your distribution reports, view the unsubscribes and remove those contacts from all your lists or just the one used for this distribution.