There are many factors to consider if your open rate seems lower than expected in your bulk email campaign. Open rates are tracked only if a recipient allows images to load in their email program.
In cases where email programs do not load images, then the open rate cannot be tracked on the results page. It is common for email programs and web-email services to block images by default unless the recipient authorizes the images to be loaded in their message.
Open rates would also be affected by the age and quality of your mailing list, as well as the content and subject line of the message as well. Messages with a good subject line and content should help reduce the likelihood that they will be caught in a spam filter (therefore reaching a greater number of recipients).