Between January and June, 3.3% of the undelivered email was routed to a "junk" or "bulk" email folder, while 17.4% was not delivered at all -- with no hard bounce message or other notification of non-delivery.

"Many marketers aren't even aware that one-fifth of their emails are never reaching the inbox," said George Bilbrey, co-founder and president of Return Path. "In many cases, marketers are seeing 'delivered' metrics that repeatedly show a 95% to 98% delivery rate."

"Many ESPs and marketers have developed the belief that whatever emails aren't bouncing have successfully reached the inbox," Bilbrey added.

"That's just not true ... Marketers need to examine their current deliverability stats, and remember that hard bounces aren't the only emails that aren't reaching your subscribers."

The U.S. deliverability rates, according to Return Path, are slightly better than Canada with an average of 82% inbox placement rate, while Canada's inbox placement rates are lower with just 75% of commercial, permission-based emails reaching consumers' inboxes.

Despite ongoing delivery issues, email marketing continues to perform as a powerful direct marketing tool. Indeed, the Direct Marketing Association just projected that email marketing will generate an ROI of $43.52 in 2009 -- twice the return earned by search and other marketing channels.

Successful deliverability to consumers' inboxes varies by ISP. Gmail, Google's email service, is the most stringent US-based ISP for permission-based marketers to reach, according to the report, as 23% of emails that marketers sent to Gmail addresses did not reach the inbox.

The top five United States ISPs ranked in order of difficulty for marketers' emails to reach users' inboxes are: Gmail, Hotmail, MSN, Comcast, and AOL.

Marketers face an even tougher time reaching business email addresses that are often protected by additional layers of email monitoring including systems such as Postini, Symantec and MessageLabs. On average, 27.6% of commercial emails sent to business addresses don't reach the inbox, Return Path found.

For this most recent report, Return Path studied data from more than 500,000 email campaigns conducted from January to June that used the Mailbox Monitor seed list system.

Return Path recorded whether the emails were missing, received in the inbox or filtered to the junk/spam folders -- for those ISPs that use such a folder. Return Path studied data from 45 ISPs in the United States and Canada.

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