Types of Content checking rules There are two types of content checking rules: Content checking rule - this rule allows to search a mail message and its attachments for certain words. This allows you to block mails and attachments with certain content. For example, you can block attachments with specific words! Attachment checking rule – An attachment checking rule allows you to block attachments with certain extensions or attachments of a certain size. Both rules can be applied to all users or a specified list of users.