A reactivation campaign will identify which addresses you can safely drop from your list without killing off live ones and re-establish connections with past customers.
How do you identify inative subscribers? Wendy Roth suggests to create a separate mailing list, and add anyone who hasn't opened or clicked on a message in, say, six months or longer, to it. Send a message with a pleading subject line, such as "We miss you! Please come back!" and include a special offer or invitation to fill out a new profile or encourage them to unsubscribe once and for all.
Move any responding addresses back to your active list. Send the message again, this time saying you'll take them off your list if they don't respond in, say, a week. Then, scratch them from your list if they don't respond. It might kill you to do that, but a smaller, more vital list will do you more good than one where nobody's home anymore.
Source: 6 tips to win back inactive subscribers (iMedia Connection)