After subject line testing, the most important thing to learn is what motivates your readers to unblock images. Image blocking has become the standard default for e-mail software and free access providers. To counteract this constraint, optimize all avenues that encourage readers to add you to their Friends List. While only a small percentage will comply, that still leaves one less group to worry about.
The next step is to test the HTML version link copy. I like the phrase "Trouble viewing this e-mail? Click here," because it doesn't use any jargon. Believe it or not, some people don't understand what "HTML" or "Graphics" or "Web Version" means. I have yet to test copy to identify the best possible phrase--but will in 2006.
We must also optimize the image-blocked e-mail version. Common sense tells me that strong, benefit-oriented HTML text in the preview pane will work best. Results from recent e-mails, however, make me wonder. A postcard e-mail, with no HTML copy at all, had a much higher click rate on the "Trouble Viewing" link than other e-mails with HTML copy. Is it possible that nothing is better? Or is it a function of the subject line? This will make for more interesting tests in 2006.
Finally, remember that open rates are a function of the reader's previous experience with your e-mail. You can test and tweak from now to eternity, but if your e-mail content falls short of user expectations, readers will cease to open it. My previous article had a few suggestions to improve your e-mail user experience. To test the impact on open rates of a better user experience, set aside a control group that gets no special treatment and compare that groups' open rates with those of groups whose e-mails have been tweaked.
With no pun intended, the goal is to get readers to be "open" to your message. If they have positive interactions with your brand, they will be more likely to take that extra step to get the best version of your e-mail, particularly if the path is easy and inviting. A Good User Experience + More HTML Reads = More Opens. Don't let your open rate decline without a fight.