Email marketing and "mobile marketing" are now effectively synonymous -- or should be treated that way. There's no trend that illustrates the decline of the PC perhaps better than the consumer shfit from reading email on PCs to mobile devices.

In Q4 roughly two-thirds of all US emails were opened on tablets or smartphones, according to Movable Ink’s Q4 2013 US Consumer Device Preference Report. That's up from 61% in Q3 and it will probably continue to grow (perhaps to 75% by year end). Although these are US data, the trend directionally applies to other developed markets.

Source: Movable Ink 

Here are some of the topline data coming out of the Movable Ink report:   

• Mobile represented 65% of email opens and desktop accounted for 35% (down from 39% in Q3)
• Roughly 16% of emails were opened on a tablet
• Apple smartphones and tablets accounted for 50% of total email opens
• Android smartphones and tablets represented 14% of all email opens (up from 10% in Q3)

Despite the steady climb in mobile email usage, far too many marketers still act as though their emails are being opened mostly on PCs. And even when HTML emails are formatted for mobile devices too often the landing pages and subsequent websites are not.