WebControl GmbH, the ".cfs" file extension is generated by the software that SmarterTools products use for its indexing. In SmarterMail, each user account has an Index folder which will contain several CFS files. Before you "reindex" next, delete all of the files within the indexing folder. It should then regenerate the necessary files. If the error persists having removing the files and reindexing, you may want to open a support ticket so we can investigate more thoroughly.