Mobile is the technology with the highest audience penetration rate globally, and with smartphone ownership on the rise, mobile email marketing is going becoming increasingly important.
With that said, the term "mobile email campaign" is a misnomer. The first and foremost concept email marketers need to embrace is that there is really no such thing as a mobile email campaign in the strictest sense. Emails are sent to email addresses, and it is the recipients alone who decide whether to read those emails on their mobile phones or on their desktop computers. Consumers' preferences can and will change based on what they are doing and what technologies they have at their disposal at any given moment. In other words, people are reading emails on mobile devices, right now, even if the marketer doesn't know about it or plan for it.

In fact, it's very likely that a high percentage of email marketers have no idea whether their emails are being read on mobile devices and, as a result, are missing the unique opportunity to communicate with their customers through emails that are both content- and design-appropriate for today's on-the-go consumers.
Marketers can safely assume that a significant portion of their emails is being read on mobile phones. So therefore optimizing emails for cross-platform performance is a large but necessary labor.
Design sensitivities for mobile email readers
Designing for the mobile web is nothing particularly new. However, it seems that mobile style-sheets haven't proven to be quite as popular in email, despite the advantages they provide to device-based viewing.
Keep in mind that some devices are better at handling HTML email. Apple and Android devices generally display HTML emails intact, making the optimization requirements minimal.
Overall though, viewing HTML email on a mobile device can be fiddly, and even on the iPhone it's possible to have text automatically rescaled to a size that's nearly unreadable or in a way that can break your design. The point is that regardless of device use, optimizing your emails for the mobile environment is essential.
A mobile screen is small -- at first glance no one's going to see much of your content, especially if it's designed to fit a PC screen instead. Also, each and every mobile device has its own unique standard settings to take into account. BlackBerry devices, for example, don't display images by default, whereas iPhones and Android devices normally do.
But how can you optimize for all these technical requirements and cater for device use across the board in one sweep?
Start by following a few straightforward, universal rules.
Here are key optimization practices that will help you produce mobile-friendly emails for most devices.
Keep it simple
A key element of mobile email design is simplicity. Most of your audience is in a hurry and needs to find relevant information to take actions on quickly. Make use of single column layouts, tighter subject lines, larger font sizes, bigger buttons for links, and an overall narrower message width. Wide emails often require horizontal scrolling, especially when there are large images involved. Keep it top-down, not left-right.
While iOS devices zoom to fit your email the width of the screen, most other mobile operating systems don't -- leaving users to resize the email or scroll left and right to view your message. Consider trimming your emails to between 320 and 550 pixels wide.
Also keep the HTML code of your email elementary -- the simpler it is, the more likely it is to be interpreted correctly and rendered well on a mobile screen. Remember: Good design is always backed by responsible coding and a platform-appropriate layout.