Delivery and open rates
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The number of emails you send out does not equal the number that get delivered. Some mails bounce, others get filtered out by email services and software, some fall foul of technical problems. The problem is you can't find out how many are really getting through.
So we can only talk about a maximum possible delivery number, which is the number sent, minus the number of bounces (which are usually registered by your mailing system).
This is the best number to use as a base when producing metrics to gauge the effectiveness of your newsletter. After all, it's a little unfair to measure the effectiveness of a newsletter that bounced.
The first such metric is the open rate, which is expressed as the number of recipients opening your email / maximum number delivered.
So if you send out 10,000 newsletters (after bounces have been calculated), and 5,000 are opened, you have an open rate of 0.5, or 50%.
In general, any newsletter getting over a 30% open rate is doing well. Anything over 50% is very good indeed. But how do you measure that open rate?
Unfortunately, it's impossible with text emails. You simply cannot discover when a subscriber opens your text newsletter.
It's a little easier with HTML newsletters. Here you can place a link to a tiny, invisible graphic in each issue. When someone opens this mail, your server will register a request to retrieve that graphic. The number of "unique visitors" requesting this graphic is the number of people opening your HTML email. Well, almost.
There are various reasons why an opened HTML email might not be registered by your server. For example, people might actually open your email when offline, or a request may not make it to the server because of technical problems, or because that graphic is blocked by the recipient's email software.
So we can talk about a minimum open rate, here, and accept that a few more people may actually open our HTML mail.
A more important problem is that opening and reading a newsletter are two different things. While you can estimate the open rate, you have no way of knowing whether the recipient then actually reads the contents.
Finally, different services and systems have different ways of reporting open rates and counting "opens" (and other metrics). Which is why any metric is best evaluated in terms of how it changes through time rather than trying to compare it to other newsletters.
NB Since this page was written way back in 2001, a lot has changed in terms of open rate theory and practice. I recommend reading the Email Open Rates Guide for the latest information.
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