A number of email administrators have complained that Office files are detected as ZIP files, and therefore blocked by their spam filters. The reason for this is that Microsoft is actually using ZIP to compress the documents.
The Office XML Formats use ZIP and compression technologies to store documents. A significant benefit of the new formats is substantially smaller file sizesâ€”up to 75 percent smaller than comparable binary documents. This is one of the advantages of using the combination of XML and the ZIP technologies for storing files. Because XML is a textâ€“based format that compresses very well, and the ZIP container supports compressing the contents, users can obtain significant reductions in file size. This type of file compression offers potential cost savings because it reduces the disk space required to store files and decreases the bandwidth needed to transport files through e-mail, over networks, and across the Web.
To allow such documents through you would have to configure attachment control to allow ZIP files through your anti-spam appliance. If you are using policies, each policy should be modified as well. You can all so educate end users not to use compress option in the "office" if they decide to send those documents out.