Do you have Symantec Mail Security, in short SMS, in your environment? Then you must have faced this problem already!
SMS considers Microsoft Office 2007 files including very common office extensions, .xlsx and .docx as executable files and delete them if it is specified as such in your email ‘Filter Policy’.
SMS email tracking log
Disposition: content compliance violation
Filter Policy: delete executable files violations
You have to trust me that I searched all ways to exclude latest Office files from this false positive but there is no patch available from Symantec and the only solution provided by Symantec is to remove .bin extension from ‘Executable Files’ policy (which you can find in SMS > Policies > Policy Resources > Attachment Lists > Executable Files).