How Can I Reactivate Unsubscribed Addresses?

You can reactivate unsubscribed addresses by going to Contacts –> Inactive Contacts and then clicking on the Type column. 

A menu will open up, with the unsubscribe type already prechecked. All you have to do is uncheck the box to reactivate a user. Note, however, that you should only re-subscribe a contact manually as an exception. This should not be a standard practice. Legally, if a contact has unsubscribed, you should not contact them again. Only use this re-subscribe action if you have a written request from a contact that they want to be on your mailing list again.

It is not possible to reactivate bounces, since these email addresses no longer accept email.