It might help if that customer installs MS Office or Libre Office, in stead of thinking Windows will do it without one of these.
I have a particular user at work who tries to email an excel spreadsheet to an external customer but they are unable to open up the file (error unknown, am still to clarify this).
However, if she forwards this spreadsheet to another business user so that they can send it instead with no changes, the customer can open it up. When she sends me the file to my external email address I can open it up.
She mentioned that there were no issues until her computer was changed, not sure when and how. But she is using the latest Office 2010 package that's all I know.
Anyone seen this before?