Email testing and tracking company Litmus published a great look at email client traffic in 2013. The upshot is that more than half of all emails are being read on mobile devices, and the trend will continue. Litmus finds a whopping 38% on iOS alone.
In response, you’d better be testing how your emails look on those devices, and doing responsive email design up front to make the best impression with all your email communications.
Email Client Market Share