When opening .msg files in Outlook 2010 you my get an error “the command line argument is not valid. Verify the switch you are using“.
This is most likely caused by incorrect file association.

Open any folder > Tools > Folder Options > File Types > MSG
Make sure it set to open with “Microsoft Outlook“.
Click on Advanced > Open > Edit
Application used to perform action: “C:\Program Files\Microsoft Office\Office14\OUTLOOK.EXE” /f “%1” (make sure path to Outlook.exe is correct)
Use DDE: Yes
Application: Outlook
Topic: System

Windows XP
MS Outlook 2010