The "subject" line is still important, but there are so many different mobile email clients and display settings that that could change; there is no way to accurately predict the number of characters that will be displayed on the phone. The best practice for mobile email is simply to put the most important information first, and don't repeat information that is already given in the "from" line.
Similarly, many mobile email clients will display the first line or so of an email as a preview, so make sure that it adds information that will make users want to open the email rather than delete it. It should be compelling and, again, should not be redundant with either the "from" line or the "subject" line. Consider each of these pieces of information a unique opportunity to cause the recipient to open the email.
2. Mobile Email Design and Coding Best Practices
Once the recipient opens the email, you don't want to disappoint. Designs that work well on both desktop computers and mobile phones are ones that are clean and simple, with a clear call to action. Featuring lot of products and prices may be daunting, especially when compressed on a tiny screen. Similarly, three columns often look too crowded on the mobile phone. It is best to stick to one hero image and call to action, then a few supplementary bits of information or marketing.
Your best chance at making an email render on the highest number of mobile devices without compromising the look of the email when it is on a desktop computer: This type of cross platform compatibility is best achieved with a flexible grid, responsive design—which essentially means that the email will adapt how it displays based on the screen size of the device that it is being displayed on. The difference between using responsive design on the Web and in email is the style sheets (CSS) must be embedded in the body of the email rather than pulled in from an external file.