The report was based on an analysis of 492 million commercial messages sent with permission to consumers around the world between May 2013 and April 2014.
The proportion of emails that didn't arrive in inboxes stayed fairly consistent throughout the time period examined, including during the end-of-year holiday shopping season when email volume typically surges.
Below, additional key findings from the report.
Inbox Placement by Country
Inbox placement rates vary significantly by country, the analysis found. For example, in the 12-month period examined, European marketers struggled more than North American marketers with missing messages but they had less mail routed to subscribers' spam folders.
Brazilian senders had the hardest time connecting with subscribers, reaching the inbox only 60% of the time, with nearly one-third of their messages disappearing altogether.
Ayaz Nanji is an independent digital strategist and a co-founder of ICW Content, a marketing agency specializing in content creation for brands and businesses. He is also a research writer for MarketingProfs. He has worked for Google/YouTube, the Travel Channel, AOL, and the New York Times.
LinkedIn: Ayaz Nanji