Even if you do everything right, there is a chance your email will have some difficulty displaying correctly for some mobile users, so be sure to include the obligatory links at the top. If possible, it is best to link to a desktop version of the email and a separate, mobile version of the email. From the mobile link, you can use user-agent detection and redirection to identify what type of phone they are on, and possibly even record that information for future marketing efforts (like promoting device-specific mobile apps). Also, don't forget that these devices are phones, capable of calling. So if you are able to take questions or orders over the phone, make the phone number prominent and clickable in the email. You might even consider including a special tracking phone number, so you can make the correct attributions in your evaluations of the email.