Velocity marketing software offers double opt-in Email marketing in addition to single opt-in Email marketing. Double Opt-In Email marketing is a good alternative to single opt-in because it is a more reliable way for you to calculate the actual number of subscribers to your Email marketing campaign. Double Opt-In Email marketing basically has an Email recipient confirm that they are subscribing to your Email list before they are actually added. This is like an added level of security and will cut down on the number of complaints you may receive.