Research Brief in their Email Blocking and Filtering Report for the First Half of 2005 says that 21% of permission-based email did not get delivered to the Inbox as intended during the first half of 2005.

The number is still high although the rate was statistically lower than the first half of 2004, when 22% of legit e-mail failed to get through. This is also the first time in more than 3 years that the rate has improved slightly. In 2002, the rate was 15% and 18.7% in 2003.

Different ISPs (in this case, email provider) have different filtering policies, so of course deliveray rates vary. The most aggressive providers were Gmail and Excite, which blocked 39% of legit email. Mac.com let through the most permission-based email, at 8%, followed by USA.net and Earthlink at 10% each.

The good news, other than the slightly better rate mentioned above, is that some of the email delivery success factors depend on the senders themselves. This fact alone means that there are things we can improve to yield better results.

Some of these factors icnlude maintaining high quality email list, implementation of email authentication standards, appropriate monitoring of delivery rates across ISPs and corporates, keeping complaint rates low, etc.

The study analyzed 140,000 campaigns sent by Return-Path new clients between January and June 2005.

Download: Email Blocking and Filtering Report.