Word OLE Web Page Storage Stream File
Microsoft Word is a word processing program developed by Microsoft and is a part of the Microsoft Office program suite.
When a word document is sent using email it is not sent as a doc file but as a MSO file which uses the .mso file extension, the mso file is embedded in to the email so that the document will be correctly rendered in the email client
» Microsoft Word
How to open .mso files?
Software to open .mso files
What are .mso files