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An unconfirmed list is a list that is set as single opt-in, rather than double opt-in. This means that when new contacts subscribe to a list, they do not receive a confirmation email that verifies their subscription.
Lists of this type are more likely to experience delivery problems because they generate more complaints and an increased number of invalid emails or get blacklisted for sending to spam traps.
Note: All contact lists created in the system are double opt-in by default, so you shouldn't have to adjust these settings unless you have disabled them or you are working with a list set up prior to 2012.