When a campaign is created in Benchmark Email, there are two versions of it: HTML and plain text. It is required that all campaigns have content for both versions so that the recipient can decide which version to view. Some of your subscribers may be using a device that does not allow the viewing of the HTML version and so by default, the plain text version will appear.
If you select the View as Webpage option in Step 1 of the email builder, this will help provide better results when recipients are using their mobile/portable devices to view your email.
Some contacts may always experience difficulty when displaying the HTML version. If this is the case, you can change the contact settings to receive the plain text version by default.