Hi!
As far as I am aware we have tested only with the latest versions of Outlook, so I am pretty sure that 2013 would not be tested seeing that there have been 2-3 newer versions of Outlook been released since then.
May I ask though, how are you getting an ".msg" file? When you schedule a WM and select the "Desktop Outlook" option, you get prompted to open an ICS fil, and if you use the "Legacy Email" option, the recipient receives a normal email with text and the links to join the meeting.