An important part of understanding how successful your campaigns are is understanding your “open” rate. According to ConstantContact.com, the open rate can be measured by how many emails are opened by how many emails are sent or bounced.
The article also goes into more detail about other open stipulations. For instance, if the email is text only, no template, no images. They also say that there is no right open number; there are many elements to consider.
You can find the article and all of the information here. If you are not receiving the conversion rates you are wishing for, the open rate could be an indicator.