Process.Start & Process.Kill

If I Process.Start a pdf file, I can later do Process.Kill and it closes
fine.  If I do the same with a saved Outlook msg file, the message opens
fine, but when I try to kill the process I get an exception because the
process has already exited.  Can someone please explain the difference,
and how I can best close an outlook form that I have opened?  

proc = Process.Start("MyPdfFile.pdf");
proc.Kill(); <== Works fine


proc = Process.Start("MyMessageFile.msg");
proc.Kill(); <== Process already exited.
 
 
Thanks for any clarification
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