Scanning of Email File Attachments for Malicious Threats
Files such as spreadsheets and word documents make it easier to hide malicious code. An anti-spam security solution should be capable of completely breaking these down, analysing all components and identifying all threats. The purpose of an anti-spam solution offers deep recursive analysis technology that is able to break down all container files, analysing each one independently and taking appropriate action via its security policies.
Zipped attachments are commonly used in order to compress files to smaller sizes. Zipped files can contain any type of files, possibly including file types not allowed through the anti-spam security solution. For anti-spam security solutions, it is a key feature to be able to unzip a file no matter how deeply nested it is, and checked for all of its content against its security policies. Also attachments such as Word documents can contain malicious embedded objects such as macros, html links, java scripts other scripts which anti-spam solutions should be able to pick up on and analyse this type of content for malicious threats as well.
Further Reading
Wikipedia's guide to Email Attachments