The cost of erroneously blocked e-mail — that sent to recipients who have given their permission to receive it — will reach $419 million in 2008, up from $230 million in 2003.

During the same period, the percentage of improperly blocked messages will drop from 17% last year to just under 10%, according to a report from Jupiter Research.

Part of the increase in wasted spending will come as marketers invest in retention-focused and sponsored e-mail messages that are blocked from recipients' mailboxes. Jupiter recommended that marketers invest in identity-based trust and bonded-sender programs; both methods increase the chance of delivery.

But these aren't the only obstacles facing e-mail marketers. While consumers reported a 26% increase in unsolicited e-mail, permission e-mail grew at a 36% rate, indicating that recipients were increasingly opting in to receive messages.

“The increase in e-mail volume and the fact that consumers are spending less time in their inbox is creating a consumer attention deficit to permission based e-mail marketing,” said David Daniels, research director at Darien, CT-based Jupiter Research. “Despite the year-over-year increase in permission e-mail, consumers only perceive an increase in spam.”