It sounds like you're referring to a spam testing platform's error for one of your email blasts. If that's correct, this error typically signifies that there is a URL (image, website link, etc.) in your email content that may be viewed as potential spam by a recipient's ISP.
If you would like us to take a look at a specific email blast, please send a link to your email, the error message in question, and the source of the error message to firstname.lastname@example.org.
Complete and current information on how to use NationBuilder is available in the documentation section.