Concept lattices assist human understanding in three ways: firstly, by collecting formal concepts that contain maximal sets of objects with shared attributes; secondly, the relatedness of concepts is revealed by providing a hierarchy of formal concepts in the information space. Finally, the concept lattice (drawn as a line diagram) reveals inferences that can automatically derive association rules. Therefore, a major hypothesis of the application of concept lattices is that they visually assist in understanding the structure of information contained within an information space. However, there has been little in the way of empirical tests to substantiate this hypothesis. This paper describes the process and results of a usability evaluation for a program called Mail-Strainer, a Web-based variant of the Mail-Sleuth program, which in turn is based on the Conceptual Email Manager (Cem).