Most consumers with smartphones use their devices to check e-mail to some degree. Digital marketing firm Knotice makes the case, and it’s one that cannot be ignored by retailers that want to get the most from their e-mail marketing campaigns.

In the first half of 2011, 20.07% of retail marketing e-mails were opened on a smartphone or tablet, Knotice finds in a study of 6.5 million retail marketing e-mails. This compares with 13.36% in the fourth quarter of 2010. What’s more, in the first half of 2011, consumers clicked on links in 11.00% of all retail marketing e-mails.

What this means is that if an e-mail is sent in its PC format, it will appear small and crunched on the smaller screen of a smartphone, making it less intriguing to consumers. Optimizing an e-mail for viewing on a mobile device—whether done through tweaks to the width of a PC e-mail or designing separate e-mails that perfectly fit the smaller screens—can lead to greater consumer engagement in a campaign.

But when optimizing e-mail and web content for mobile users, it’s important to optimize the entire experience, not simply how the e-mail message itself looks on a phone’s screen, Knotice says.

“Marketers need to be thoughtful about how the message is rendered, but more importantly, how the user can take action in the most convenient way possible,” the firm says. “Whether that means clicking through to a mobile-optimized site, tapping on a phone number to call a customer service agent, or even users providing their e-mail address to have a shopping cart, wish list, product information or follow-up reminder sent to them so they can complete the action when more convenient. Focus needs to be on optimizing the e-mail as well as the post-click experience in tandem for an overall satisfying user experience.”

Knotice studied more than 150 million marketing e-mails in 11 industries. 20.24% of all marketing e-mails are opened on a mobile device, according to the study. Knotice breaks down mobile opens by device. 12.78% were opened on an iPhone, 3.92% on an iPad, 3.15% on an Android device, 0.22% on the now-defunct HP webOS (formerly Palm), 0.05% on a device running one of the Windows mobile operating systems, 0.01% on a BlackBerry and 0.11% on other devices.