Almost a quarter of permission based email marketing messages don't make it to the inbox says a new study from email deliverability services company Return Path. According to the Inbox Placement Rate Benchmarks Report, 22 percent of marketing emails sent to subscribers who had opted in did not reach the inbox during the first half of 2013. ReturnPath looked at a sample of almost 1 trillion messages sent worldwide, and found that 18 percent of all email messages were either blocked or went missing, while the remaining four percent hit the spam folder.