Forrester research is predicting email spend to "balloon" to $2bn (£1.2bn) by 2014, a nearly 11% compound annual growth rate.
Falling CPMs, a high return on investment, and growing consumer use of social email accounts will fuel the use of email by direct marketing professionals, the company said in its annual Email Marketing Forecast.
David Daniels, Forrester analyst, said: "By 2014 direct marketers will waste $144m on emails that never reach their primary target. Successful direct marketing pros will alter their tactics to overcome inbox clutter and increase relevancy."
The study also found that retention email - email that recipients have "blessed" with their permission - will continue to replace paper communications and will make up the largest share of marketing messages.
Retention emails will account for more than one-third of all marketing messages in consumers' inboxes by 2014, representing increased competition for marketers.
While the bulk of the market will continue to deploy email marketing on a self-service basis, the growing complexity associated with data integration and new tactics to increase relevancy will drive healthy grow in use of email service providers, research found.
Spending on ad-sponsored newsletters will also double over the next five years as traditional print publishers face falling circulation and ad revenue.
Daniels said: "The use of email in social networks will be one of the biggest challenges for direct marketers. Over the next five years, marketers must bridge the gap between social and traditional inboxes with social sharing tools."