4. Moving targets are hard to reach

Reading email on mobile phones is cumbersome enough. Add in the images and links so prevalent in email marketing, and they can become unreadable. Even on smartphones, a single link can take up several lines, causing people to hit the delete key.

However, Forrester's Katz thinks that mobile email usage will actually increase as smartphones become standard. Email marketers need to work with service providers to understand how their messages will look on different devices, she says.

Benner of VerticalResponse advises email marketers to use alternative text whenever possible, in case mobile recipients have images turned off. "Make sure the important links go at the top, and include a link to view the message on the website," she says.

5. Email is too complicated and not snazzy enough

Consumers respond strongly to rich media, and online video came into its own in 2008. Zenith OptiMedia's global ad spend forecast, released in April, predicts 29.8 percent growth in internet video and rich media spending in the U.S. The pressure for email marketers to up the glitz collides with the deliverability and mobility issues we've already examined.

Of course, email service providers are constantly adding new bells and whistles. In April, Goodmail Systems released CertifiedVideo, a service that lets marketers embed streaming video in their emails. In the press release, the company promised, "Consumers can now watch videos within their email inbox without having to click to an external website."

Jennings points out that video in email has been around since 2000. "The problem back then was bandwidth. You had to do as many as eight different versions, depending on how fast the computer was, etc." With the profusion of email devices and clients, the challenge is just as great. Nevertheless, she thinks video emails will come sooner rather than later -- at least, it won't take another nine years.

That said, she advises marketers to remember that it's all about effectiveness, not glamour. "You have to have a good reason to use this technology."

Susan Kuchinskas is a freelance writer who has written for Adweek, Business 2.0, M-Business and internetnews.com.