The idea of sending an opt-in (or "re-opt-in") campaign to subscribers to verify email permission is not new, but interest in these campaigns is increasing. Over time, a portion of your email list will become unengaged -- which has several negative effects. Unengaged subscribers result in lower response rates and wasted marketing dollars. Re-opt-in campaigns are useful for cleaning old or unengaged subscribers off your list by confirming which subscribers want to continue receiving marketing emails. This results in a healthier list and increased return on investment.