"Everyone knows their best customers are at Gmail, so that's a problem," said Quinn Jalli, senior vice president of digital marketing services at Epsilon, a digital marketing firm. "It's less clicks from the most affluent portion of society, so it has a disproportionate impact on the bottom line for retailers."
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Google announced Thursday that it would automatically show images in Gmail messages (before, Gmail required an extra click to display images, largely because malware could be embedded in the images.) Google now stores images on its own servers, where it screens them for safety, instead of on the senders' servers.
The flip side for retailers is that by storing images on their own servers, they received much more information about Gmail users who opened their messages, such as their locations. Now, they know someone opened an email but not much more.
Half a year since Gmail introduced tabbed inboxes, fewer emails from retailers are being opened, according to three services that manage mass emails. And while some data showed that Gmail users nonetheless spent more money — perhaps because they proactively sought out retail emails in the promotions inbox — other data showed the opposite.
(Read more: Storms walloped retail traffic, ShopperTrak says)
Gmail shoppers, who tend to be wealthier and more tech-savvy, clicked on retail emails 14.5 percent more than Yahoo users before the change to Gmail, according to Epsilon. But by October, the difference had shrunk to 4.2 percent, indicating that Gmail users were ignoring marketing emails more often.
Accordingly, Epsilon's retail customers have reported a decline in revenue from Gmail users. And even when people did not open retail emails, the messages reminded people of the brand by showing up in their inboxes, and that effect is now lost, Mr. Jalli said.