More and more emails are being opened on mobile devices first. According to one study desktop clients are falling by the wayside while mobile clients are being used 20%+ more.
Products can really shine on a mobile phone and customers can zoom in on the details. In addition phone numbers in the body of the email can be clicked on making it easy for customers to connect with your business for answers to questions.
Mobile customers are also more likely to share and pass on the emails too, many people handle social media on their phones.
So while you design your emails for desk top clients, web mail clients like Google, Yahoo, and Hotmail, keep those phones in mind---they're the future.