Though most email marketers think email content plays the biggest role in inbox delivery, a Return Path study shows that 83 percent of email delivery problems are instead the result of reputation issues.

Nearly 20 percent of legitimate commercial email does not get delivered to the inbox, but typical content concerns such as the word "free," images, coding, or spelling only accounted for 17 percent of delivery issues according to the Return Path research (PDF) based on 550 email campaigns. Instead, 77 percent of delivery issues occur because of the sender's email reputation, Return Path found.

Moreover, an additional 6 percent of the time the inclusion of a reputation-challenged domain included in the email content caused delivery issues.

Return Path found that on average 77.3 percent of problems were directly due to the sender's reputation. "Notably, with six of the primary ISPs, the rates of reputation filtering were even higher - at 97 to 100 percent," according to George Bilbrey, VP and general manager of Return Path's Delivery Assurance division.

Five of the leading ISPs showed reputation to be 100 percent of the reason behind non-delivery. Of the ISPs that showed content to be a significant filtering trigger, 89 percent are known to use Brightmail, which filters on numerous components, including the reputation of the actual content as based on spam traps and complaints previously attributed to the same content.