I just got off the phone with a client who was very concerned about her open rates. We occasionally come across this from a new mailer and it can be hard to explain to them why they shouldn't worry about it. The reason we provide an open rate statistic is, for the most part, because people expect it. Having been in the email marketing industry for almost 20 years, we know that open rates aren't really an accurate way to measure the success or failure of a campaign.
At one time, open rates were an accurate and useful way to record campaign data, but starting in 2004, Microsoft began blocking images by default in their email programs. In the proceeding months and years, other email clients followed suit. Today, nearly every email client provides their users with the option to block images and most do it by default. This dramatically changes the usefulness of open rate statistics. They no longer provide accurate information about the individuals who open email. Certain situations, such as if an email happens to have no pictures, and therefor provides no incentive for your users to click the "load images" button, makes open rates appear even worse. This isn't to say that you should include images solely for the purpose of getting your open rates up. What you should do is accept that open rates just aren't as important or as useful as they once were.
Open rates, while no longer effective at projecting how many people have actually read your email, can sometimes be used effectively as a meter stick to compare results. Looking at the open rates of two different campaigns sent to the same list and noting how they differ can be a helpful exercise. Each list you send to will have a baseline open rate based on several factors. Those factors will include: the email habits of your recipients, what email client they are using, and how interested they are in your general message. Once you have figured out this baseline open rate, you can use it to judge the overall effectiveness of future mailings. If one promotion or newsletter performs above or below the baseline, you know you've either done a superior job with that content, or missed the mark.
If you want a more accurate statistic to monitor, you may want to keep a close eye on the clickthrough statistics. A clickthrough is recorded anytime someone clicks on a link in your email. This number has nothing to do with unreliable image loading and can't be distorted by the email client. It's simply a one to one representation of how many people have clicked the links in your email. Many of our clients are trying to drive traffic to their website, this number is actually the most crucial number for them. If you have a call to action on your email that involves clicking a link, the clickthrough statistics will give you actual numbers on how many people answered that call to action. Clicking on a link or acting on the mailing is the true test of it's effectiveness and should be of much greater concern then open rates.