Relevance has emerged as king in legitimate email marketers' battle against spam and junk mail buttons to reach prospects' inboxes. Knowing that their relationship with prospects and customers is only one click away from being terminated, these marketers understand the need for valuable, timely offers in messages where the content is as carefully crafted as the readers are segmented and selected.

But, too often, the unique needs of the mobile user are not sufficiently considered. At the first stages of building an email campaign, it is also important to remember the rapidly increasing number of mobile users taking advantage of high-speed networks and mobile devices to get their news and communications on the go.

Here are four top rules to follow when trying to email prospects and customers on the road:

1. Talk Their Language

Many mobile devices still cannot read HTML so make sure to create a text version of each email. Some email marketing service providers have a "sniffer" feature that will automatically detect email clients that cannot accept HTML and present them with the text version.

2. Keep it Short and Sweet

The mobile devices that do accept HTML, however, are still different than desktop email clients. Whether the mobile device in question accepts HTML or only text, the messaging for mobile users should be shorter; the smaller screens will work best with emails about 60 to 65 characters wide.

3. Reach a Moving Target

Because mobile device users are on the run, a particularly compelling, branded subject line becomes even more important. Like all sent emails, sometimes the subject line may be the only thing a recipient is going to read, but a mobile user is likely to be more distracted, less patient. Can your subject line compete with the distractions surrounding that person, whether they are sitting in an office, waiting in a line, or standing in a crowded airport? Make sure the subject line is short, with no more than 30 characters maximum.

4. Give Them the Option

As the number of mobile device users continues to grow, you may want to have a mobile option on your list subscription forms and create separate emails that you optimize for mobile devices.