The cost to online marketers of emails wrongly blocked as spam will almost double in the next five years, according to US group Jupiter Research.

In 2003, failure to deliver commercial messages sent with the permission of the recipient cost marketers an estimated $230 million (€182m; £125m). This figure is expected to grow to $419m in 2008.

That said, the actual proportion of permission-based emails blocked as spam is expected to decline from 17% to 10% over the period, reflecting improvements in ISP systems and sender authentification schemes. However, total spend on this form of commercial email will rise so rapidly that the amount wasted on wrongly blocked messages will continue to rise.

Jupiter believes marketers should look at trusted sender schemes as a way to ensure their mail gets through. It also points to changes in the way recipients are dealing with sponsored messages.

"The increase in email volume and the fact that consumers are spending less time in their inbox is creating a consumer attention deficit to permission-based email marketing," commented research director David Daniels. "Despite the year-over-year increase in permission email, consumers only perceive an increase in spam."

Data sourced from: Jupiter Research; additional content by WARC staff