Convert MSO to XLS

An .mso to .xls conversion is not possible. Commonly, you might encounter Microsoft Office Macro Reference Files (.mso) as email attachments (in cases where senders use MS Word, or similar programs, as their default text editor for Microsoft Outlook. 

Microsoft Office Macro Reference Files cannot be converted to anything, and can't even be opened. These files are only visible when the email recipient doesn't use Microsoft Outlook. They contain special formatting for the email's text body.