Some 20.5 percent of permission-based commercial email was either delivered to
junk mail folders or not delivered at all in the second half of 2005, according to a research brief (pdf) from Return Path, MediaPost writes. The non-delivery rate varied by internet service provider (ISP), with Excite and Gmail filtering out the most emails (42.9 percent and 40.4 percent, respectively) and Earthlink and Mac.com filtering out the least (7.8 percent and 8.1 percent, respectively).

In the first half of 2005, non-deliver was 21 percent; it was 22 percent in 2004 and 18.7 percent in 2003. Corporate filters MessageLabs, Postini and Brightmail didn't deliver 30.5, 24.0 and 21.5 percent of permission-based commercial emails, respectively.

For its research, Return Path examined some 117,000 permission-based email campaigns sent through 28 ISPs and the three leading corporate filtering systems.