Historically, it has been difficult to measure the effectiveness of marketing campaigns because target markets cannot be adequately defined. Email marketing carries the benefit of allowing marketers to identify returns on investment and measure and improve efficiency. Email marketing allows marketers to see feedback from users in real time, and to monitor how effective their campaign is in achieving market penetration, revealing a communication channel's scope. At the same time, however, it also means that the more personal nature of certain advertising methods, such as television advertisements, cannot be captured.
Over time, the popularity of email marketing campaigns resulted in personal email inboxes that were overflowing with the latest and greatest offers. Email campaigns grew and people grew tired of them. Laws that allowed customers to unsubscribe breathed new life into email marketing campaigns as customers could easily pick and choose the companies they were interested in.