On average, 12% of email subscribers read email messages via mobile device during the standard workday, but that level increases to roughly 19% in early morning, late afternoon, and the weekend hours, according to a new study by eROI.

Optimizing messages for mobile is most critical for email campaigns delivered on days immediately before and after the weekend, because mobile's peak use is between 4 pm Thursday and 9 am Monday, according to eROI.

Below, other findings from the eROI study titled Email Marketing, On the Go, which examined how the subscriber bases of 20 brands interacted with email during a 30-day period.

Email Clients/Devices Used

Email content is now delivered to a wide range of clients and devices. Microsoft Outlook is still the most common email client (31%), followed by Yahoo Mail (15%), Windows Live Hotmail (11%), and Apple Mail (10%).

The Apple iPhone accounts for 9% of email interaction and the Google's Android accounts for 3%.

Looking for solid, substantiated information about email from the industry's best resources? The 67-page Email Marketing Factbook featuring 39 charts tells you about email usage, what captures users' attention, and how organizations are using email to reach customers. This email factbook consists of chapters 1 & 2 from the larger Digital Marketing Factbook a 144-page compilation of data and 110 charts that also covers search engine marketing and social media.

Email Rendering Agents

Subscribers are also using a growing variety of rendering agents. Internet Explorer is most common (49%), followed by Safari and Chrome (26%) and Firefox (15%).

About the data: eROI partnered with Litmus to measure how email subscribers of 20 big-brand e-commerce, retail, and publishing companies interacted with email. Findings are based on roughly 90,000 email messages sent in August 2010.