On average, 21 percent of permission-based emails did not reach the inbox during the first half of 2005 (relatively unchanged from the first half of last year) because they were either blocked or filtered into the junk folder, according to a new email deliverability study from Return Path, reports ClickZ. Senders' deliverability problems stemmed as much from their own practices (e.g., low list quality and number of complaints against the sender) as zealous blocking of emails by ISPs; blocking rates for individual mailers were as high as 54 percent.

The highest delivery rates for legitimate mail were for Mac.com, which filtered or blocked only eight percent, and Earthlink, which blocked 10 percent; on the other hand, Gmail (39 percent blocked), MSN (33 percent) and Hotmail (32 percent) blocked the most.