If there were any lingering doubts about the magnitude the impact of mobile is having on e-mail marketing and deliverability, they’ve just been dispelled by new numbers reported by ReturnPath earlier this month.

The deliverability services firm reports that mobile penetration has increased a whopping 80% since October, due in large part to the growing popularity of tablet computers (like the iPad and iPad2). In addition, the new breed of smart phones running operating systems like Apple iOS, Google Android, and Microsoft Windows Mobile seem to be well on their way to dislodging the dominance of non-HTML-capable phones (like some older models of RIM’s venerable Blackberry franchise) in many enterprise deployments.

The new numbers confirm the trend expounded upon in remarks by Google’s Executive Chairman Eric Schmidt this past winter, in which he observes that mobile adoption is “happening faster than all of our predictions”. E-mail marketers can act now to optimize their content to meet the mobile wave, before it can wash away hard-won gains in open rates and ROI.

Recently, Real Magnet published a whitepaper of best mobile practices, which provides specific, tested steps you can implement right now to ensure your mail looks just as appealing and is as usable on mobile devices as it is on the desktop. Here are few tips to get you started:

Keep it simple: Designing for mobile devices is challenging because of the wide variety of operating systems and hardware devices available. Ultimately, senders are tasked with creating a single message that will work well with all of them. The best rule of thumb is to keep the design simple and clean. The more sophisticated the design, the greater the chance it will break.

The width of your email should be 100%: The content should occupy the full width of the recipients’ mobile devices without requiring them to scroll horizontally. To achieve this, code the outermost container of the message using a 100% width. This ensures that the e-mail will wrap properly, given the mobile e-mail application’s width constraints.

The width of images in the message should be 300 pixels or fewer: Smartphones render images in e-mail with arguably more inconsistencies than desktop or webmail applications. Some smartphones will automatically scale the images so they fit within the display screen perfectly, while others will zoom the entire email out in order to keep the images at the designer’s intended proportions. Play it safe: use smaller images so that they will be seen at the size they are meant to be seen and not force horizontal scrolling if they are downloaded at actual size.

Leave plenty of space between links: Touch-screen users will need to click links with their fingers, and many mobile devices display the e-mail zoomed out by default (e.g. iPhone). This leaves very little room for error in clicking, so leave ample space between links to ensure that your recipients can easily identify and click without accidentally clicking the wrong link.

Don’t use complex images that must fit together seamlessly: In order to design graphically interesting emails, many designers will slice complex images to fit precisely within the confines of a table structure. Commonly used examples are rounded corners or tabbed navigation. The more complex the layout, the less likely it will render correctly across various mobile devices.

Mobile design can be a real challenge, given the all of the different screen sizes and functionality available on mobile platforms and devices. But with some careful planning and design choices, senders can ensure that their messages can keep up with their recipients, where ever they may go. The full text of the whitepaper is available, free for the clicking.