Top ISPs and web-based email providers did not deliver 21 percent of permission-based emails to consumers’ inboxes during the first half of 2005, according to a new Return Path email deliverability study. In 2004, 22 percent of permission-based email was not delivered to consumers’ inboxes.

Return Path’s study monitored a sample size of 140,000 marketing and transactional campaigns sent by clients via Return Path’s Delivery Assurance Mailbox Monitor service between January and June 2005. While an average of 21 percent of emails were blocked or filtered and did not reach consumers’ inboxes across all mailers, the levels of blocking and filtering varied by mailer from a low of 1 percent to a high of 54 percent. On an ISP by ISP basis, the non-delivery rates ranged from a low of 8 percent to a high of 39 percent.

Read more about the opt in email marketing issues revealed by the Return Path study at http://www.crm2day.com/news/crm/115982.php.