Error message when you use Word 2007 to try to open a mail merge document that was saved in an earlier version of Word: "The Office Open XML File cannot be opened because there are problems with the contents"

When you use Microsoft Office Word 2007 to try to open a mail merge document that was saved in an earlier version of Microsoft Word, you receive an error message that resembles the following:This problem occurs if the following conditions are true:

• The mail merge document was saved in the .docx file format.
  
  Note This problem does not occur if you open a mail merge document that was saved in the .doc file format.
• The mail merge document uses a separate header source.

When this problem occurs, you may experience the following symptoms:

• If you try to recover the contents of the mail merge document after you try to open it, you may receive an error message that resembles the following:
  Microsoft Office Word has encountered a problem and needs to close.
• If you manually find the data source of the mail merge document, Word 2007 opens the mail merge document. Additionally, Word 2007 uses the data source as the header file and the header file as the data source.