New York—More than 20% of commercial, permission-based emails are failing to reach the inboxes of intended subscribers in the United States and Canada, according to a new study by email solutions company Return Path.

These emails reached only 79.3% of inboxes during the first half of 2009, according to the company's "Deliverability Benchmark Report." Of the undelivered email, 3.3% is routed to a “junk” or “bulk” email folder, but 17.4% isn't delivered at all, with no hard bounce message or other notification of non-delivery, the company said.

While such personal email accounts as Gmail, Hotmail and Comcast screen email rigorously, marketers face an even tougher time reaching business email addresses that often are protected by layers of email monitoring. On average, 27.6% of commercial emails sent to business addresses don't reach the inbox.

Return Path's study monitored data from its Mailbox Monitor service, tracking delivery, blocking and filtering rates for more than 500,000 email campaigns from January to June 2009.